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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 IV.

THE second most essential thing after a good light, and a successful illumination of the object, is a compound lens, so far corrected for spherical and chromatic aberration as to reproduce on the ground-glass an image in which straight lines are exhibited straight, and all the parts, both in the central and peripheral portions, are clearly defined and free from spectral colors. No single lens can be practically ground and polished so as to be free from spherical aberration; which means that no lens can be constructed so that, with the whole opening, the rays both through the center and all the way to the edges shall be refracted to one point. The focus of those rays which are transmitted through the lens near the periphery, is nearer to the lens than of those which pass through the center. Hence exist a multiplicity of foci, thus converting that which ought to be a point into a circular space; and that which ought to be a line, into a rectangular or curvilinear space; hence the origin of indistinctness and haziness in the photograph-the picture is devoid of sharpness and fine definition. If the optician were able to grind lenses with ellipsoidal surfaces, then a single lens might be constructed so as to be totally free from this sort of error or aberration. This, however, is manifestly a practical impossibility. The form of lens which distorts the least, that is, which has the least spherical aberration, is the one which is well known as the crossed lens, whose radii of curvature are in the proportion of one to six. Spherical aberration may be corrected partly by a combination of lenses and partly by the use of diaphragms, the latter of which exclude all but the central rays, or all but the peripheral rays.

Chromatic aberration arises from the difference in the refrangibilities of the colored rays in the spectrum, and the decomposition of white light into the colored or spectral light, whenever it is transmitted through a homogeneous transparent medium whose two surfaces are not parallel. But the two surfaces of a lens are never parallel; therefore every simple and homogeneous lens must decompose light into the spectral colors of which the violet on one side is much more refrangible than the red on the other. On this account the focus of the red light will be more remote from the lens than that of the violet light. This sort of aberration, therefore, has the same tendency as spherical aberration to convert points and lines into circular, rectangular, or curvilinear spaces, with an additional inconvenience arising from the different colors, which it is well known are possessed of very different degrees of actinism. Now, when both these causes of distortion and indistinctness exist in a lens or in a combination of lenses, it is not in the power or skill of the photographer to obtain a well-defined, sharp, and actinically well-developed picture. Some sorts of glass refract light more than others; again, some decompose light into the spectral colors differently, so that the angle between the extreme rays, the red and the violet, where the refracting angle of the prism or lens is the same, but the material different, is not a fixed quantity. Combining these angular differences, the differences in the refracting powers of transparent media and the varying radii of curvature, mathematicians are now able to devise a variety of combinations of lenses which are practically free from the aberrations in question. Generally crown-glass and flintglass are combined in accordance with the principles just alluded to. Such a combination corrects partially; it is a decided improvement over any single lens as regards fine definition; but what it gains in definition it loses in magnifying power. A triplet, or a combination of three lenses, properly constructed, is an improvement upon the doublet; and a pair of doublets whose radii and distances are mathematically and optically calculated, can be made to produce more correction than it is possible to obtain from a triplet. Three pairs, too, will effect more than two; but, unfortunately, whatever is now gained in focal sharpness is diminished in value by the absorbing power of the different lenses; so that when the combinations increase in number, the light which finally emerges, however much corrected, becomes more and more actinically weak. For photographical purposes, a pair of compound lenses can be constructed and adjusted so as to be practically perfect. We are indebted to Dolland for the first achromatic combination. Doublets and triplets are decidedly the best arrangements for landscape photography; whereas two pairs of doublets, adjusted at a given distance apart, or at a variable distance apart, are preferred for portraiture. The nearer the pairs of combinations approach each other, the greater the magnifying power; the maximum power existing when they are in juxtaposition. When a tube is fitted up so that one of the combinations admits of motion by a rack and pinion, its focal length can be thus changed, and is practically good within certain limits. With such tubes, too, it becomes an easy matter to adjust a pair of them for stereoscopic purposes.

The following rules and information will be found useful for ascertaining the comparative value of the different tubes in the market.

To find the Principal locus of a Lens.--Fix the lens in a tube or aperture in the camera; then turning the camera to the moon, adjust the slide until the image on the groundglass is perfectly in focus; measure the distance from the ground-glass to the nearest surface; then with a pair of callipers take the thickness of the lens and divide this thickness by two; now add this half to the first distance, which will be the focal distance exactly if the lens is double-convex and its radii of curvature are equal. Proceed in like manner with a compound lens; the result will be very nearly correct. Where the tube contains two pairs of combinations, a similar method may be adopted without much error. In speaking of the focal distance of a lens, or of a combination, it is customary simply to measure the space between the ground-glass and the nearest surface of the last combination, after focussing the moon or the sun.

To find the Equi-distant Conjugate Foci of a Lens or Combination.--Adjust the object, as, for instance, a cardpicture, in front of the lens or combination in the camera, until the image on the ground-glass is of an exactly equal size with the object when in perfect focus. Measure the distance from the image to the object and divide this distance by two; the quotient will be the quantity required.

To find the Comparative Value o f Two Lenses or Combinations which produce the same Sized Image of an Object at the same Distance.--Take the difference between the equidistant conjugate focus and the principal focus of either lens; the smaller this difference the better the lens, because the focal depth or penetration is greater; that is, objects farther apart can be brought into focus consentaneously and with more facility when this difference is small than when it is large. If this difference were zero, a lens would be perfect.

To find the Magnifying Power of a Lens or Combination.--On a sheet of card-board, in the middle, construct a circle one inch in diameter, for instance; place this sheet on a table. Insert the lens or tube into a piece of wood placed horizontally over the circle, and raise or depress it by blocks or books until the circle is seen most distinctly when viewed with one eye. Now, by a little practice, with both eyes open, one looking through the tube and the other on the side upon the paper, marks can be made on the board at the extremities of a diameter of the magnified circle; because the eye which is free can, by sympathy, see the magnified image which the other eye beholds, and the pencil at the same time. After this, measure the distance between the pencil-marks, and divide this distance by the diameter of the real circle; the quotient will indicate the number of times the image is larger than the object, which number is the magnifying power.

To find the Comparative Magnifying Power of Lenses or Combinations.--Measure the distance in either between the lens and the ground-glass when the moon is in focus, or measure the size of the image; the greater this distance or image, the less the magnifying power. The quotient arising by dividing one distance with the other will give the amount of magnifying power in favor of the lens, whose distance is the shorter.

To find a Single Leas equivalent in Power to a Compound Lens.--If a compound lens and a single lens be placed so that their centers are at the same distance from the moon or a distant object, for instance; then, if they produce the same sized picture, one will be equivalent to the other. (For further information vide chapters on Microphotography and Microphotography.)

To ascertain whether a Combination is corrected for Spherical Aberration.--Draw two parallel straight lines, exactly an inch apart, and two or three inches long, on a piece of card-board. Move the slide until they are correctly in focus on the ground-Mass, and until the width between the lines is two inches. If this distance remains the same, that is, if the lines do not deviate from straight lines and from parallelism, the combination is aplanatically correct; if, on the contrary, the images of the straight lines are curves, the spherical aberration has not been corrected. Apply a diaphragm of small opening in front of the combination; it will be perceived that the curvature of the lines will diminish as the aperture diminishes. If with a very small aperture the lines are still curved, the combination is worthless; whereas, if the lens or combination can be used without a diaphragm and still produces straight and parallel lines in the Images, such a magnifier will be very valuable.

To ascertain whether a Lens or Combination is corrected for Chromatic Aberration.--Adjust the slide most accurately, so that the image of an object is very clear and distinct. Next see that the surface of the collodionized plate is exactly coincident with the ground-surface of the glass, that is to say, at the same distance from the nearest surface of the lens. Sensitize the collodion film and take a picture. If, when developed and fixed, this picture is as sharp and well-defined as it was on the ground-glass, the lens is achromatic; if, on the contrary, the contrast between light and shade is imperfect, and the definition and sharpness feeble, the combination has been either over-corrected, under-corrected, or not corrected at all. The. actinic rays are on the violet side whose refrangibilities are greater than those of the red rays; their focal distance, therefore, is shorter. Focus again, and after this has been accomplished draw the slide containing the ground-glass outward about one sixteenth part of an inch, insert the sensitized plate, expose, develop, and fix, as before. If the picture is better than before, it shows that the actinic focus is longer than the luminous, and that the combination has been over-corrected. By proceeding in this way, it can be ascertained exactly how much the slide has to be drawn out in order to produce a picture as sharp as that on the ground-glass. After this distance is found, the groundglass has to be advanced or sunk deeper in its frame by this amount, whereby the camera becomes adjusted to the tube. Should it happen that the slide has to be pushed in after focussing in order to obtain sharp definition oil the collodion, it is an indication that the lens is tinder-corrected or not corrected at all. Where a lens requires no adjustment of the ground-glass, it is said to be achromatically correct, or that the actinic and luminous foci are coincident. The value of a lens in this respect is inversely proportionate to the amount of adjustment required; that is, the greater the amount of adjustment, the less its value.

Other methods have been proposed to test the coincidence of the actinic and luminous foci. One consists in pasting a newspaper on a flat board, and erecting the latter perpendicular to the horizon and in front of the opening of the lens, so that the axis of the lens passes through the center of the newspaper and at right angles to it. The operator next obtains a sharp focus upon the central parts, and afterward obtains a positive of the object. If the central parts are still in focus in the picture, the combination has been achromatically corrected; if the parts intermediate from the center to the periphery are in focus, the lens has been over-corrected; and more so if the marginal portions alone are in focus; whereas, if the picture is nowhere sharp, it is probable the lens has not been sufficiently or not at all corrected for chromatic aberration.

A second method is to focus first in the ordinary way; then, placing a piece of violet-colored glass in front of the lens, to focus again; if the two foci coincide, the actinic and luminous foci coincide.

A third method is that proposed by Claudet, which consists in placing printed cards at short distances apart, as, for instance, of one tenth of an inch, in grooves on an inclined plane resting on a table in front of the tube. Let there be five cards so arranged, and focus upon the middle one. If the first or second is in focus, the lens is under-corrected; if the middle one is sharp, the lens is unexceptionable; and if the fourth or fifth is well defined, the combination is overcorrected.

For an over-corrected lens or combination the ground-glass has to be set back by introducing thin pieces of card-board between it and the ledge of the slide in which it rests; and where the correction has been defective, the glass has to be sunk deeper as before mentioned.

If a combination has been thoroughly corrected, I throw aside the ordinary ground-glass slide entirely, and focus upon a piece of glass of the same size as the collodionized plate, and introduced into the self-same aperture which is to contain the negative. In this way the collodion-surface and the ground-surface must necessarily coincide.

How to buy a Good Lens.--Do not purchase a secondhand tube of any one, if you are a beginner in the art of photography; but throw yourself implicitly and in full confidence into the hands of a photographic house of decided reputation, who will furnish you with a lens and camera in perfect adjustment and in working condition. The tubes manufactured in this country by two or three different firms, are not inferior to the best from abroad; and the advantage yon have in dealing directly with them or their immediate agents is, that if by chance a lens turns out in any way defective, you can immediately obtain redress by an exchange. As soon as an operator is sufficiently skilled in optics and their application to the heliographic art, he will be in a condition to rely upon his own judgment, and to make his purchases where pecuniarily they are the most advantageous. The best criterion by which to ascertain whether, after purchasing an adjusted tube and camera, the actinic and luminous foci coincide, is to take the plate-holder containing a plate of glass with the slide drawn and place it upon a table, collodion side uppermost; by the side of this place the groundglass slide with the ground-surface uppermost. Placing a rigid flat ruler over either of these, it will be easy to measure the distance from each glass surface to the edge of the ruler. Where these two distances coincide, there has been no need of adjustment; and the lens may be regarded as good. If the difference is well marked, I would recommend you to return the tube and get a better.

Supposing, furthermore, lenses to be aplanatic and achromatic, there exist special differences by which their relative values can be distinctly estimated. The value of such instruments depends upon the extent of picture in perfect definition which can be obtained by them, with a given opening, focal distance, and diaphragm, and on the velocity with which this work can be accomplished. If of two lenses of equal opening and equal focal distance, the one will produce as sharp and large a picture without a stop as the other can with a diaphragm; the former is very much superior, because, with much more light, the operation of actinism will be relatively quicker. In like manner, if of two lenses whose three parts, as enumerated above, are all equal, but the picture of one is considerably larger than that of the other, and in every respect as well defined, the comparative value is easy to determine. Wherever this difference in the size of the picture exists, other things remaining the same, it will be found that the lens which produces the larger picture will likewise comprehend a larger angular space containing objects. Drawing imaginary lines from the two extremities of the landscape, for instance, through the center of the lens or combination, to the corresponding extremities of the picture, two isosceles triangles are formed with their vertical angle at the center. This angle or opening of the two outside rays constitutes what is denominated the angular aperture of the lens. The greater this angle, the other values remaining the same, the greater the practical worth of the lens. For the purposes of portraiture, the lenses in general have but a small angular aperture, and produce a picture but little more in diameter than half the focal distance. The relation between the opening of the lens, the aperture in the diaphragm, the focal distance and the diameter of the picture, as given in the Chimie Photographique, are as follows: Calling the focal distance unity, then the diameter of the lens will be 1/5 of this unity, that of the stop 1/40, and that of the picture 3/5 If the diameter of the distinct picture is equal to the focal distance, the angular aperture will be about 53°; and if this angle be 90°, the diameter of the picture will be about twice as great as the focal distance. It is asserted that the new globe-tubes, the invention of C.C. Harrison, have an aperture of ninety degrees, and that they are free from spherical and chromatic aberration; they will therefore be in a condition to produce large pictures with a small focus. The only disadvantages which they probably possess will be a deficiency of light, owing to the smallness of the aperture in the stops; an inequality of action from the center to the peripheral parts; and the production of what is denominated the "ghost" on the center of the picture, owing to reflections between the lenses of the combination. For architectural and landscape photography they must be inestimable, if the assertion of their merits is true.

The firms in this country that have gained a well-earned reputation for the manufacture of portrait, etc., lenses are those of C. C. Harrison & Co., and of Holmes, Booth, and Haydens; in Great Britain, those of Ross, Dallmeyer, Grubb, etc.; in France, of Jamin, etc.; in Germany, of Voightlaender, etc.

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