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Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XXXVIII.
PRINTING OF TRANSPARENT POSITIVES BY THE DRY PROCESS.

POSITIVES on glass, whether for the stereoscope or the magic lantern, that is, such as are to be regarded by transmitted light, are prepared most easily, most quickly, and most effectually by the Dry Process. the first part of the operation consists in obtaining a correct negative of the object, either by the wet or the dry process, the latter being preferable, because the negative so obtained is less liable to be damaged in the subsequent manipulations than the ordinary unvarnished collodion negative. the negative in question is required to be very sharp in all its parts, moderately dense in the deepest shades, though not so much so as for the ordinary printing on paper, and transparent in the lights. the film must be thin, bright, and free from all deposit of dust arising from reduction or impurities. the negative best adapted for the printing of glass transparencies is incontestably that with albumen; for it requires no varnish, and is endowed with all the requisites above mentioned. the albumenized glass, too, is the best for the reception of the transparent image. Dry plates by the Tannin Process are the next best; it is a good plan in this instance also to have the negative an albumen print, and the transparencies on tannin plates.

Provided with such a negative, place it in the shield of the plate-holder with the image toward you; on this place a sensitized tannin or albumen plate, the film being from you, so that the two films lie in intimate juxtaposition; close the door, whose spring retains the plates firmly in contact. Introduce the plate-holder into the grooved receptacle at one end of the cylinder, as described in a previous chapter of this work, expose the other end to the light of a cloud, etc., and draw the slide. an exposure of a few seconds will be all-sufficient. the precise time can not be accurately given, but is easily ascertained with given materials. Begin with an exposure of one second, and proceed until you find the time best adapted for the circumstances. With dry plates, it is not absolutely necessary to use the cylinder; the cylinder, however, yields superior results.

The development of the plate depends upon the nature of its constitution; if an albumen plate, develop it accordingly; if a tannin plate, in like manner. These different modes are given in detail in the preceding chapters on the subjects in question; as well as every other information referring to the completion of the picture after development, such as washing, fixing, drying, and varnishing.

The color of an albumen print is not sufficiently agreeable for stereoscopic purposes. This color is improved by immersing the plate in the first place in a dilute solution of bichloride of mercury, and after it has been washed, in a solution of sel d'or, (the double hyposulphite of gold and of soda,) when the color will be an agreeable sepia tone. Chloride of gold alone, in dilute solution, communicates to the fixed positive an agreeable purple tone; naturally the prints have to be washed always after such operations.

To take Copies of any given size.

Where the required transparency must be of a given size, as is the case in the preparation of slides for the magic lantern, and for other similar exhibitions, or for church windows, the printing has to be performed in the camera and by means of the lens. This process is described in a preceding chapter of this work.

Theoretically a picture can be made as many times larger or smaller than the original by an analysis of the well-known formula for the conjugate foci of a double convex lens. This formula is as follows:

Symbol

where the thickness of the lens is not taken into consideration; but with this consideration, the formula will be:

Symbol

When any two of the preceding terms v, f, and u, are known, the third can be found; f signifies the principal focal distance, u the distance of the object from the nearest surface the of the lens; v is the distance of the picture on the ground glass from the same surface; t is the thickness of the lens; r the radius of curvature of the first surface; and p is the index of refraction of the transparent medium of which the lens is formed.

Without going into a minute optical discussion, I will analyze the first formula so as to be enabled with a lens of a given power, and with a given sized object to show what must be the respective distances of the object and image from the lens.

In the first place I will explain a few technical terms, such as the axis of a lens, the optical center of a lens, the principal focus of a lens, the conjugate foci of a lens, the equivalent focus of a combination.

The axis of a lens is a line perpendicular to all the diameters drawn from edge to edge.

The optical center of a lens is the point where a line (joining an impingent and an emergent ray that are parallel to each other) crosses the axis; this center is sometimes within the lens, sometimes on its surface, and sometimes external to it.

The principal focus of a lens is the point where parallel impingent rays converge and cross after refraction and emergence; it is the burning point of the sun's rays. the distance of this point to the optical center is called the principal focal distance.

The conjugate foci are any point on an object and its corresponding point on the image. the distances of these two points to the optic center are denominated conjugate focal distances; these distances, however, are generally reckoned from the vertex or surface of the lens next to the object.

The vertex is that point where the axis touches the surface of the lens nearest the object.

The equivalent focus is a term that refers to compound lenses, such as those used by the photographer; it is the principal focus or the focus of parallel rays of the combination. It is called equivalent from being compared with a single lens that will produce the same sized picture at the same distance of the object. If rays from an object impinge upon a lens and on emerging converge, they will cross each other, and where they cross they will form a picture of the object.

The axis of a radiant point, that is, of any point on an object, does not mean the same thing as the axis of the lens; it is a line that is broken at the two surfaces of the lens, passing through the optic center, of which the impingent and emergent parts are parallel. On this axis the image of the object is found. If rays emerge parallel, they will never cross, and therefore produce no picture; if they diverge after emergence, the image will be on the same side of the lens with the object, and is denominated a virtual image.

Equidistant conjugate focus refers to an object and its image on the ground glass when they are equidistant from the optical center, or more intelligibly speaking for the photographer, when the image and the object are of the same size. the distance of the equidistant conjugate focus can be derived from the principal focal distance, or vice versa. Thus in the equation:

Formula

Let f=12 inches, required the value of v and u when they are equal, or when the picture and object with the lens in question are of the same magnitude? By transposition

Formula

Therefore if a given single lens has a principal focus of 12 inches, the ground glass as well as the object will have to be placed respectively at a distance of 24 inches from the lens in order to obtain a picture of the same size as the object.

The principal focal distance of a single lens can be found

with sufficient accuracy for all practical purposes by measuring the distance of the lens from the burning point, and by adding to this distance half the thickness of the lens.

The principal focal distance of a combination can be found with the same degree of accuracy by adjusting the camera before a given line so that the image of the line on the ground glass is exactly of the same size. One fourth of the distance between the object and the image is the principal focus required. For instance, let this distance be 48 inches, then v is 24 and u is 24 inches; by substitution

Formula

The distance of either the image or the object from the optical center bears a direct ratio with the size of the image or the object, whether the lens be single or compound. Thus then, if we know the respective linear magnitudes of the image of the same object as obtained by two single lenses or by a single lens and a combination, as well as the principal focal length of the former, (which can always be easily obtained by the sun's rays,) we can by the legitimate proportion derive the principal focus of the other single lens or the equivalent focus of the compound lens. for instance, let the principal focal length of a single lens be 3 inches, and the linear magnitude of an image of a given object be 2 inches as obtained by this lens; let also 5 inches be the linear magnitude of the image of the same object at the same distance when taken by another lens; required the principal focal length of the other lens, (if single,) or the equivalent focal length of the combination?

By proportion as Formula the principal focal length required. in the proportion Formula , let u be n times larger than v. required the proportion that/bears to u?

Formula

Hence if we multiply the principal focal length of any lens by one more than the times the image is linearly greater than the object, we shall obtain the distance the screen is to be placed from the lens; and if we divide this latter product by the number of times the image is linearly greater than the object, we obtain the distance of the object from the lens. in these analytical conclusions we suppose the lens to be single and very thin. the deductions thus derived have to be regarded in reference to the center of the combination. the following table has been constructed in accordance with the preceding principles, and it exhibits the distances between the object and the lens, the image and the lens, and the object and the image. Any degree of reduction and enlargement with a given lens or combination, whose equivalent focus is known, can be effectuated with great ease by adjusting the object and the ground glass at the distances indicated.

O in the following table stands for the distance between the object and the center of the combination.

I stands for the distance between the image and the center of the combination.

S stands for the distance between the object and the image, or the sum of the two preceding.

TABLE of the distances between the Object and the center of the Lens; the Image and the center of the Lens; and of the Sum of these distances.

TIMES OF REDUCTION OR ENLARGEMENT.

Focus of
Lens.
Reduction
Distances.
1 2 3 4 5 6 7 8 9 Enlargement Distances.
Inches   Inches Inches Inches Inches Inches Inches Inches Inches Inches  
1 O.
I.
S.
2
2
4
3

4
1 1/8
5 1/8
5

6
1 1/5
7 1/5
7
1 1/6
8 1/6
8
1 /17
9 1/7
9
1 1/8
10 1/8
10
1 /19
11 1/9
I.
O.
S.
. O.
I.
S.
3
3
6


6
2
8

1 7/8
9 3/8
9
1 4/5
10 3/5
10½

14¼
12
1 5/7
13 5/7
13½
1 11/16
15 3/16
15
1 6/9
16 2/3
I.
O.
S.
2 O.
I.
S.
4
4
8
6
3
9
8
2 2/3
10 2/3
10

12½
12
2 2/5
14 2/5
14
1 1/8
16 1/8
16
2 2/7
18 2/7
18

20¼
20
2 2/9
22 2/9
I.
O.
S.
O.
I.
S.
5
5
10


11¼
10
3 1/8
15 5/8
12½
3 1/8
15 5/8
15
3
18
17½
2 11/12
20 5/12
20
2 6/7
22 6/7
22½
2 13/16
25 5/16
25
2 7/9
27 7/9
I.
O.
S.
3 O.
I.
S.
6
6
12
9

13½
12
4
16
15

18¾
18
3 /3/5
21 3/5
21
3 3/7
24½
24
3 3/7
27 3/7
27
3 3/8
10 3/8
30
3 1/8
33 1/8
I.
O.
S.
O.
I.
S.
7
7
14
10½

15¾
14
4 2/3
16 2/3
17½

22¼
21
4 1/5
25 1/5
24½
4 1/12
28 7/12
28
4
32
31½
3 15/16
15 7/16
35
3 8/9
38 3/9
I.
O.
S.
4 O.
I.
S.
8
8
16
12
6
18
16

21¼
20
5
25
24
4 4/5
28 4/5
28
4 2/3
32 2/3
32
4 4/2
36 4/7
36

4-½
40
4 4/9
44 4/9
I.
O.
S.
O.
I.
S.
9
9
18
13½

20¼
18
6
24
22½
5 5/8
22 1/8
27
5 2/5
32 2/5
31½
5 ¼
36¾
36
5 1/7
41 1/7
40½
5 1/6
45 9/16
45
5
50
I.
O.
S.
5 O.
I.
S.
10
10
20
15

22½
20
6 2/3
26 2/3
25

31¼
30
6
36
35
5 5/6
40 5/6
40
5 5/7
45 5/7
45
5 5/8
50 5/8
50
5/59
55 5/9
I.
O.
S.
O.
I.
S.
11
11
22
16½

24¾
22
7 1/3
29 1/3
27½
6 7/8
34 3/8
33

39½
38½
6 5/12
44 11/12
44
6 2/7
5- 2/7
49½
6 3/16
55 11/
55
6 1/9
61 1/9
I.
O.
S.
6 O.
I.
S.
12
12
24
18
9
27
24
8
32
30

37½
36
7 1/6
43 1/6
42
7
49
48
6 6/4
54 6/7
54

60¾
60
6 2/3
66 2/3
I.
O.
S.
O.
I.
S.
13
13
26
19½

29¼
26
8 2/3
34 2/3
32½
8 1/8
40 5/8
39
7 4/5
46 4/5
45½
7 7/12
53 1/12
52
7 3/7
59 3/7
58½
7 5/16
65 13/16
65
7 2/9
72 2/9
I.
O.
S.
7 O.
I.
S.
14
14
28
21
10½
31½
28
9 1/3
37 1/3
35

43¾
42
8 2/5
50 2/5
49
8 1/6
57 1/6
56
8
64
63
7 7/8
70 7/8
70
7 7/9
77 7/9
I.
O.
S.
7.5 O.
I.
S.
15
15
30
22½
11¼
33¾
30
10
40
37 ½
9 3/8
46 7/8
45
9
54
52½

61¼
60
8 4/7
68 4/7
67½
8 7/16
75 15/16
75
8 1/8
83 1/8
I.
O.
S.
8 O.
I.
S.
16
16
32
24
12
36
32
10 2/3
42 2/3
40
10
50
48
9 3/5
57 3/5
56
9 1/8
65 1/8
64
9 1/7
73 1/7
72
9
81
80
8 8/9
88 1/9
I.
O.
S.
8.5 O.
I.
S.
17
17
34
25½
11 1/8
45 1/8
67
11 1/8
45 1/8
42½
10 5/8
53 1/8
51
10 1/5
61 1/5
59½
9 7/12
69 1/12
68
9 5/7
77 5/7
76½
9 9/16
86 1/16
85
9 4/9
94 4/9
I.
O.
S.
9 O.
I.
S.
18
18
36
27
13½
40½
36
12
48
45
11¼
56¼
54
10 4/5
64 4/5
63
10½
73½
7210 2/7
82 2/7
81
10 1/8
91 1/8
90
10
100
I.
O.
S.
9.51 O.
I.
S.
19
19
38
28½
144
42¾
38
12 2/3
50 2/3
47½
11 7/8
59 3/8
57
11 2/5
68 2/5
66½
11 1/12
77 7/12
76
10 6/7
86 6/7
85½
10 11/16
96 3/16
95
10 5/9
105 5/9
I.
O.
S.

Application of the Preceding Table.

If the equivalent focus or principal focal length of a combination be known, it is very easy to arrange the object to be photographed, the camera and the screen, so as to produce a picture so many times larger or smaller than the object, as may be required; for instance, let the focal length of the combination be 4½ inches, what must be the conditions of the three things, object, combination, and ground glass, so as to obtain an image eight times larger than the object?

Look for 4½ in the first vertical column, and for 8 on the first horizontal line; where these two columns meet will be found all that is required. in the first place the object and the ground glass must be 45 9/16 inches apart, the ground glass is 40½ inches from the middle of the combination, and the object is consequently 5 1/16 inches from the same point.

If we wish to diminish the size of the picture eight times, then the two latter of the above terms are inverted, the object being 45 9/16 from the center of the combination, and the image only 5 1/16 inches from the same point.

The table can be extended as far as desired, by using the multiples of the numbers already given. If we required the conditions for 15 inches focus, multiply those along column 5 by 3, the results will be the conditions required.

Microphotography and Macrophotography.

This branch comprehends the mode of taking photographs of microscopic or almost invisible objects, as also of amplification by means of the solar camera. in either case means are resorted to by which light can be concentrated or condensed on the object or collodion positive to be copied, and enlarged or diminished. These means are combinations of plane reflectors, concave reflectors, double convex or piano-convex lenses. the appendages to the solar camera and to the solar microscope are facsimiles of each other; but the solar microscope existed before photography had been elicited from chaos; the solar camera, therefore, is a mere imitation of its antecedent; the patentees of the latter instrument, then, can make no claim to originality of design; their only claim can be the application of the instrument to photography.

Solar Microscope.

The appendages to the solar microscope, that is, the condensing part of the apparatus, consist in the first place of a plane mirror in the form of a rectangle, whose width is at least equal to the diameter of the plane-convex or double convex lens, which condenses the light received from the mirror. the length of the mirror must be about four times its width. At one end there is a hinge-joint, which allows the mirror to swing on the same like a door. the hinge is fixed to a circle of brass or other metal, which, by means of a dentated periphery, admits of a circular motion. By this contrivance it will be seen that the mirror has two motions at right angles to each other; for instance, supposing the back of the mirror faced the sun at noon, and were perpendicular to the horizon, then one of the motions mentioned would cause the mirror to incline toward the sun, until finally it would be flat on the horizon. the other motion permits the mirror to move either toward the East or the West; so that, as it now stands, if moved toward the West, the silvered surface would face the setting sun. By combining these two motions consentaneously, the mirror can always be so inclined as to reflect the rays of the sun from rising to setting into the axis of the condenser. the two motions in question are effected by means of screws and pinion-wheels, etcetera.

The part just described might be a concave mirror admitting of the same motions; this would act as a reflector and condenser at the same time. the condenser is fixed in the brass plate which is attached to the window-shutter, and around the condenser the metallic ring moves, to which the hinge of the mirror is attached. the object of this part of the apparatus is, by refraction, to cause the large bundle of parallel rays that impinge upon its surface, to be condensed from a cylindrical into a conical form, so that at a given distance this converging and condensed light will arrive at its apex or focus.

Now, at this focus, all the light that has passed through the lens will be concentrated; and at a variable distance, before it arrives at this focus, it will cover a variable space, varying from a point or zero upward to an amount equal to the surface of the lens.

The amount of condensation will be the ratio between the squares of the distances from the focal point; thus, suppose the focal distance be twelve inches, and that we intercept the cone of light at three inches from the focus; then by dividing the square of twelve by the square of three we obtain the ratio, which is sixteen, and this indicates that the light at this distance is sixteen times more intense than it was when it first immerged from the lens.

The object of the refracting lens, therefore, is to illumine the object with light. This is the primary view of the matter, but it does more than this; each ray from the condenser not only illumines each point on the transparent object upon which it impinges, but on emergence after refraction it passes on modified by the medium through which it has penetrated, and carries, so to say, this part of the picture with it; the cone of modified light is in fact the picture set in motion, and so directed as to strike the surface of the camera-lens which is next to it. These rays are convergent, and are each the axis of an independent cone of divergent rays from each illumined point of the transparent negative. Some photographers maintain that the axes alone (that is, the rays that constitute the cone of light from the condenser) are available, and that the divergent rays around each axis are of no avail. This, however, is a mistake, and is equivalent to saying that, if an opaque object were illumined by a condenser or reflector, the picture could be taken only by focussing the cone or the beam of reflected light; whereas we know full well in copying that the rays that enter the camera through the lens, and that go to the formation of the picture, can not be any of the reflected rays, because these are perpendicular to the surface of the copy, and would indicate that the impingent rays were also perpendicular, which is an impossibility, owing to the opacity of the camera and its tube, which occlude all perpendicular rays. On the contrary, each illumined point becomes a new radiant, from which proceeds a divergent pencil of rays, of which many around the axis are refracted by the lens and brought to a focus on the other side.

If the condensing lens be achromatic, the light will be white; if not achromatic, it will produce spectral colors, of which some are useless in photography, whilst others are exactly those which are needed. Now the scientific optician can arrange his non-achromatic condenser in such a manner, in reference to the lens and the negative, as to make use only of the violet light, or the actinic part of the spectrum, for the formation of the picture. the focus of the violet or actinic light is shorter than that of the luminous or yellow part.

The next appendage to the solar microscope is the object-holder, which has a sliding motion to or from the condenser, in the neighborhood of the focus, by which means the object can be placed in a condensed part of the cone of light, which is just sufficient to cover it and no more, a contrivance by which light is economized.

The remaining part of the instrument is the microscope proper, which contains the corrected objective for magnifying the object.

Now the above description is precisely the same as that of the condensing part of the solar camera. With such an arrangement of mirrors and refractors, the camera and screen may remain fixed during the whole time of the operation.

Another arrangement for concentrating light is accomplished by means of reflectors fixed in the form of a frustum of a pyramid. But in the application of this contrivance the camera and screens must all move together on a universal joint, like a heliostat, by which means the silvered surfaces of the reflectors can always be preserved in front of the sun, so as to catch his rays, (as described in a previous chapter of this work.)

The mode of using the solar microscope and the solar camera is in no wise different, excepting that in the former a transparent object is substituted in the holder for the transparent collodion negative in the latter. Each is placed in the cone of condensed light, in order to be brilliantly illumined, and in such a position, in reference to the objective or photographic lens, as to bring the focus of the actinic rays immediately on the optical center of the last or front lens of the combination. It is by this means alone that the best enlarged picture can be obtained.

How to find the point where the Lens is to be placed.

It appears then that the lens may not be placed in any position for maximum effect; the true position depends upon the power of the condenser, in combination with the power of the posterior lens of the tube, where such is used. There must be a relative connection between these two powers; but this is not maintained in any of the solar cameras in the market, from the fact that tubes are not considered as parts of the solar camera; operators are consequently left to apply whatever combination they may have on hand; we must therefore avail ourselves of what is next best, and fix the combination where the maximum effect can be obtained with given materials.

Knowing the length of the principal focus of the condenser and its diameter, as well as that of the compound lens from the posterior lens, the mathematician can easily calculate how much the former focus will be shortened by the interposition of the tube. Supposing, for instance, the diameter of the condenser be eight inches, and its focal length be twelve inches, then the angle which the side of the cone of condensed light makes with the diameter will be 71° '.23 Moreover, let the diameter of the posterior lens be two inches, and the focal length from the back lens two inches, then the angle formed between the side of its cone and the diameter will be 63° 45'. That is, if the rays entered the combination parallel, they would form a cone, of which the outside ray would have this angle with the diameter of the back lens. But, being interposed in the cone of condensed light, of which the rays are convergent, the tendency of the combination is to shorten the focal length, by reducing angle 63° 45' to 56° 00', the difference between these angles being the same difference that exists between 71° 32' and 63° 45'. As the angle diminishes, so will the focal length of the cone of condensed light be diminished, and in the present instance to the amount of half an inch.

Besides this, we have to reduce this distance still more, in order to find the actinic focus, which the mathematical optician can easily find.

But the generality of photographers are not supposed to be in a condition to deduce the requisite corrections in this way; we must therefore show by practical means how we can approximate to the same results.

Ascertain the focal length of the condenser by finding the distance of its burning point from the glass; then, when the tube is screwed out to the extent of its play, measure the distance from the face-plate, in which the tube is fastened, to the front lens; subtract this distance from the focal length of the condenser, the difference will give the distance of the condenser to the outside of the camera nearly, or to the part upon which the face-plate of the tube is to be screwed. More accurately the same result can be obtained by interposing the tube in the condensed light, and by moving it backward and forward, until the focal or burning point is just on the outside of the front lens; let an assistant measure this distance from the outside of the camera, and at this distance fix the tube permanently. Whilst doing this the greatest care is required to make the axis of the condenser coincide with the axis of the tube.

This is the first rude adjustment. the second adjustment consists in bringing the actinic focus so as to coincide with the optic center of the front lens. Screw back the sliding part of the tube and turn on the sun; the luminous focus will be quite visible in the dark space behind the camera. Now insert a piece of deep violet-colored glass between the condenser and the objective, so as to intercept all the colors of the luminous cone, excepting the violet, and ascertain where the violet cone comes to a focus; screw the tube out until this focus is just in front of the anterior glass; then, knowing the thickness of the front lens, advance the tube until the blue focus is in the middle of the front lens, and let this be the final and permanent adjustment of the tube in reference to the condenser. Mark this position by a line on the brass work, in order that the tube can be adjusted at a moment's notice when required to be used.

The negative-holder is movable by means of a screw, so that it can be brought into focus upon any screen on the other side of the tube. Whenever this operation of focussing is to be performed, insert the violet-colored glass, so as to focus in reference to actinism, and not to luminosity. By this means the luminous picture on the screen (that is, when the violet-colored glass is removed) may not be quite sharp, but the printed picture on the paper will be sharp and beautifully defined. The same mode of proceeding may be followed with the ordinary camera, where there is any doubt of the correction of the tube for actinism. Place in front of the tube a piece of violet-colored glass every time you focus.

Macrophotography, or the Art of Taking Enlarged Photographs. The Negative for Enlargement.

The size of the negative will have to depend on the diameter of the condenser; if this be nine inches, a one-sixth plate will be large enough, the object being to get the negative as near the apex of the cone of concentrated light as possible, and in such a position as to be totally covered by the cone.

The Quality of the Negative.

The negative suitable for the solar camera must be very bright, well defined and quite clear. The glass must be thin, perfectly flat, or in the same plane and homogeneous. The negative effect need not, in fact, must not be carried on to the same extent as for positive printing; it is but a trifle in advance of the ambrotype; if there should happen to be the slightest quantity of fogging, that is, reduction on the transparent parts, it will be necessary either to take another negative or to clear off the fogginess. This is effected by flowing the plate with a dilute solution of iodine in iodide of potassium, until the picture turns slightly cream-colored; the plate is then washed and flowed with a solution of cyanide of potassium, which dissolves the newly formed iodide of silver and thus clarifies the picture. As soon as the latter is satisfactory, as to brightness, cleanness, and fine definition, wash and dry the plate, but apply no varnish.

As soon as the negative is in its place, and accurately focussed actinically, fix the prepared paper on the screen in its place. In order to preserve the paper perfectly flat and smooth, sponge the back with a wet sponge, and after it has thoroughly expanded, and lies uniformly, and without undulations, go round the edge to the amount of half an inch on the same surface which has been sponged with a thick solution of gum-arabic; attach the paper so prepared to an even plate of glass or drawing-board, of somewhat smaller dimensions than the paper, and allow it to dry. When dry, all the corrugations and undulations will have disappeared; the paper will be smooth and flat, and ready to receive the image, supposing naturally it has already been sensitized in the silver bath. If this operation has been neglected or omitted, the silver solution can be very expeditiously poured upon the surface and spread with a pad or tuft of cotton wool, until the film is uniform. The excess of silver is then removed, and the plate is reared on one corner over a wineglass to receive the drippings.

When dry it is placed in the focus of the negative, and the sun is turned on. By means of the two screws on the solar camera, the sun's light is maintained in its position during the whole operation. Printing on albumenized paper by the solar camera is a tedious operation, requiring sometimes several hours before it is complete, and sometimes even a clay or two by reason of the cloudiness of the sky. Where this sort of printing is practicable, as is the case generally in our own country, the results are the best. Printing by development, however, is more reliable, because it is altogether independent of the condition of the sky, whether cloudy or cloudless.

Several processes for printing by development will be found in the chapter in which this subject has been discussed. I will insert another in this place, from its applicability and reliability. It is the process of Blanquart-Evrard, whose prints have been so much admired.

Bromo-iodizing Bath for Paper.

Water, 12 ounces.
Gelatine, 1 drachm.
Iodide of potassium, 1 drachm.
Bromide of potassium, 15 grains.

Immerse the papers in this bath, as many at a time as it will contain, and keep them there for two or three hours. The bath can be used over and over again until exhausted. The papers are then taken out and hung up to dry. As soon as they are dry they may be preserved in a portfolio for use.

Previous to being sensitized they are exposed for a quarter of an hour to the vapor of hydrochloric acid. This operation is easily effected by fixing the paper along the sides and under the lid of a large nearly air-tight box, by means of varnished pins. At the bottom of the box place a saucer containing a handful of halt, an ounce or two of sulphuric acid, and Half as much boiling water. Vapors of hydrochloric acid will be generated in abundance, and will thus saturate the paper.

Sensitizing Bath.

Nitrate of silver, 1 ounce.
Distilled water., 14 ounces.
Nitric acid to give it an acid reaction.

Let the paper float in this bath for ten minutes. By decomposition they will now contain the iodide, bromide, and chloride of silver. After sensitization they are allowed to drain, and then dried either by pressure between folds of bibulous paper or by suspension in the dark-room.

The exposure required will vary from a couple of seconds to half a minute beneath a negative, and longer than this on the screen of the solar camera. When the image is just visible, the printing has been carried on long enough.

Development.

The picture; is brought out by immersing it in the ordinary gallic acid bath, at a temperature of 80 degrees, and by keeping it there for a quarter of an hour or more as circumstances require. The bath must; be large enough for many pictures at a time; these are kept in motion all the while. They assume a disagreeable color, and become covered with spots which are removed by the operations afterward. As soon as the depth of shade is sufficiently intense, the: prints are taken out, laid one by one on a glass plate, and sponged on boll) sides and then immersed in a bath of hyposulphite of soda for five minutes, in which they are toned.

Hyposulphite of soda, 1 ounce.
Rain-water, 20 ounces.

After this they are removed direct into a second bath of hyposulphite of soda of the same strength, and are allowed to remain for twenty minutes, in which they are completely fixed.

The prints are then carefully washed in several waters and finally immersed in a bath of dilute hydrochloric; acid, which removes a yellow deposit and the spots above mentioned. A second washing completes the operation, with the exception of drying and exposing to the action of light for several weeks, which improves the reddish tone by changing it gradually into purple.

These prints will keep for an indefinite time, although toned with sulphur.

Microphotography, or the Art of taking Diminished Copies of Photographs, or Photographs of Microscopic Object's.

Diminished Photographs-It is a much easier operation to diminish the size of a photograph or object by photographic means than to amplify one; and the result in general is more satisfactory, because all the errors of the original are diminished in the same ratio as the whole picture is diminished. In order to take portraits so invisibly small as not, to be seen without the aid of a magnifier, we require a small camera specially arranged for the purpose. Such cameras furnished with the necessary objective, are manufactured by Bertsch in Paris. The tube requires no focussing; the only condition to be observed is to place the photograph, object, or print to be copied at or beyond a given distance. All lenses hive this property of requiring but one adjustment, which is permanent when once found, for objects beyond a given distance, which varies directly as the focal distance or power of the lens. Lenses for the diminutive pictures in question are in loons for all distances beyond three feet or so. Objectives, such as are sold for microscopic purposes, whose focal distances are one inch, half an inch, or a quarter of an inch, stay easily be arranged in a very small camera to take these diminutive portraits. But very little ingenuity will suffice to make such a camera out of a small telescope, where one tube slides into another. In the end of the inner tribe the objective is fixed; in the end of the outer, the ground glass and the plate-holder. This compound tube is fixed permanently upon a solid support six inches high, on a piece of board four or five feet in length or even more. On the opposite end of the board a plane is erected at right angles to the former and also to the axis of the camera. Find the point on this vertical board where the axis cuts the sauce, and mark it as the center of the picture to be copied. The picture is fixed upon this plane by means of tacks or pins in an inverted position and so that its center coincides as near as possible with the mark just made.

The next proceeding is to focus the lens. Take the long board and place it so as to receive the sun's rays upon the picture. Now move the inner tube of the camera in and out until the image is seen on the ground glass by means of a powerful magnifier. Focus with the greatest sharpness. This operation is very refined and requires a great deal of patience. When the utmost definition is thus obtained, place before the opening of the tube a piece of very thin violet-colored glass and see if the image is still sharp; if it be, fix the two tubes permanently so that their relative position can not be changed. In future this operation of focussing is no longer required. If, however, the picture is not sharp when the violet-colored glass is interposed, focus until you get perfect definition, and then fix as just directed.

The glass to receive the picture is thin and homogeneous; it is flowed also with a very thin collodion and sensitized as usual. All the operations are precisely the same as those already described in the preparation of the ambrotype. Of course a pair of spectacles of very high magnifying power is required while developing, fixing, and mounting. With a pair of pliers or forceps the small piece of glass can be broken down so as to fit into the ring, etc., which is to receive the picture.

The objectives manufactured by Grunow in New-York for microscopes have succeeded quite well with me in the production of almost invisible pictures; and I have no doubt be will be able to fit up a microscopic camera for such as require one from the indications here given. Such a camera, requiring great refinement of workmanship, will of course be snore likely to be better made by those who are accustomed to the refined adjustments of a microscope than by the photographer himself. The objectives of Grunow are not only unexceptionable, but are endowed with qualities superior to those in many of foreign origin.

Microscopic Objects.--The objectives just alluded to are very well suited for taking enlarged photographs of microscopic objects, such as the porous structure of wood, the siliceous deposit in guano, blood corpuscles, starch granules, itch insects, etc. Such an objective is fixed to an ordinary bellows camera, so arranged on a sliding platform that the axis of the objective coincides with the axis of the cone of concentrated light from the condenser of the solar microscope. The latter instrument has a special opening between the condenser and the objective to receive the transparent object whose photograph is to be taken of an enlarged size. If the objective is not quite achromatic, insert a piece of thin violet-colored glass over the object while focussing, and fix the objective so that the violet cone of light terminates in the optic center of the objective as before described. Focus by means of a pair of very powerful spectacles or a compound microscope. In the first place make the camera firm on the platform, when the objective is once in its place; then draw out the ground glass nearly as far as it will go, and afterward move the microscopic object nearer or farther off, as the case may be, by means of the thumb-screw, until the picture is visible on the ground glass; finally focus with accuracy so as to get perfect sharpness. The violet-colored glass may now be withdrawn. The prepared collodion plate is inserted in the place of the ground glass; the slide is drawn out, and the sun's light turned on for a fraction of a second. It is in many instances an advantage to keep the violet-colored glass in its place, because it moderates the light; and the result is even better with it than without it.

Finish the plate for a positive or negative according to rules already prescribed in ordinary photography.


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