It may be found that the amount of spherical aberration is so great that no amount of changing will bring it to the proper standard. One means of partially overcoming it would be the decrease in the size of the lenses, which would mean that they will admit less light, and consequently be slower.

The thickness of the lens which causes the distortion must be accurately determined. However, we will assume that the various corrections are finally combined as far as is possible, and the skill of the optician must determine to what extent they will be. It may be possible to combine the spherical and chromatic aberrations sufficiently to make the former acceptable, and a new series of experiments will have to be undertaken based on a combination of new glass.

After a lens which meets the ideal of the optician is completed, it may be said, "We can understand that the first lens has involved a great amount of work, but after the completion of the sample none of these difficulties will arise." To a certain extent this is true, but it must be remembered that the optician's work is almost entirely empirical. Each lens is an experiment; there is an individuality in each which cannot be overlooked, A lens is rarely if ever perfect when completed, and each one is a special study. There are certain features which rarely if ever change when working under the same formulas and using the same glass, such as coincidence of visual and chemical foci, centreing, and. size, but others, such as homogeneity and refrangibility of glass, sphericity of surfaces, and length of focus are variable, and often can only be reached after repeated trials, and in many instances cannot be obtained at all.

One point of the utmost importance is the sphericity of surfaces. To simply grind and polish a lens is exceedingly simple; to do so and obtain spherical surfaces is extremely difficult. Instead of being spherical, a surface may be and often is a series of elevations and depressions. They may be in the shape of rings, or may be confined to spots or radiate from the centre as pronounced as the spokes in a wheel. Heat, even when moderate, when applied to a certain portion of the lens, causes the same effect. There are optical tests by which you can easily see the expansion of glass after the finger has been allowed to rest upon it for a moment. The same effect may be obtained by fixing the lens in the mounting.

Often lenses are so distorted by the pressure exerted on them in the mounting that it seems almost impossible that they can form even a fair image. What is the effect when these conditions exist? First of all, there is a lack of sharpness over the plate, and then depth of focus. An elevation on the lens will give a shorter focus for all those rays which fall upon it, because of its shorter radius than the focal point for the entire surface; whereas a depression will cause a longer focus, so that instead of all the rays combining in one point there are a series of focal points. When depth of focus is caused by reason of this fault, it is to be condemned, because it is at the sacrifice of definition, and any lens possessing it cannot be rated as good. Depth of focus when caused in this manner is one of the qualities which can be determined without comparative tests, and every lens purchased should be tested to this end. A good means to determine the proper correction of the surfaces is by the use of a diaphragm. Focus for a sharp image on the ground glass by means of a magnifier with a full opening of the lens, and note this point in the camera.

Then rack out of focus and insert a diaphragm sufficiently large that the image can just be distinguished, and again focus by means of the magnifier. If the objective is perfectly corrected, this point will coincide with the first, and if not, it is, as stated before, a serious fault, because there will be a different focus for each diaphragm.

Depth of focus and speed are incompatible, inasmuch as both qualities depend on the aperture of lenses, assuming of course a relatively large diaphragm and focal length. While the former decreases with the increase in the aperture, the latter increases. Depth of focus is no doubt a very desirable quality, but to obtain it in a legitimate way with high speed is simply impossible. In two lenses of similar construction, of the same diameter and focus, it will be exactly similar, and the ratio of depth will increase with the decrease of the diaphragm. In view lenses, depth of focus can almost always be obtained by the use of diaphragms, and this is also true in objectives specially made for portraiture; but in these the same is obtained by placing two systems farther apart or varying the distance of the individual lenses, which means nothing more nor less than the production of spherical aberration.

Flatness of field and distortion in lenses are variable quantities, and, although depending to a certain extent on the diameter of lenses, exist to a greater or less degree in one or the other lens, according to the amount of skill and care of the optician. The tendency almost since the first use of lenses has been the increase of speed and consequent increase of aperture of lens, and although the advance has been and is slow to the uninitiated, it means an enormous amount of work to the optician. The actual work of grinding and polishing is comparatively trifling, as is shown in the cheaper form of lenses, where the cost of glass is just as high, but wherein the lenses are used whether good or bad, unless they have faults easily noticed. The time-consuming and worrying work is that of continually testing and correcting the lenses.

As to the homogeneity of glass. A disk should be of the same consistency throughout.. When it is not, it possesses either impurities or air bubbles which are usually easily seen, or so-called striae - which are not so discernible, but of greater importance. These are caused by the unequal consistency of the glass or unequal annealing, and have a similar appearance to a glass of water when strong sugar water is poured into it and is not well mixed, or heated air as it rises from a stove or hot ground. All glass not specially made for optical purposes has them, and can easily be seen by looking through it edgewise, even in the best plate. Of course no disk is used which shows them or air bubbles or impurities on a preliminary examination. They often appear before the lenses are completed, and work then ceases; but very often they cannot be detected until the lenses are finished. Air bubbles can easily be seen by any person; when they exist, the optician must judge whether they are sufficient to impair the sale-ability of a lens.

If, however, a man who has a reputation to guard permits it to pass, then the purchaser should not be hasty to condemn it, for it then usually possesses merits which far outweigh this disadvantage. Take for instance an air bubble as large as a pin head in an ordinary size lens. Its area is so infinitesimally small as compared with the area of the lens that it is of no account whatever, and does not affect the lens in the least.

Striae, however, which are difficult to detect, are positively injurious when existing to any extent, inasmuch as they refract the rays passing through them in different directions, thus preventing them from combining to form the image, and they sometimes may cause a false light. They often pervade an entire lens, and occasionally can be seen with the naked eye, but as a rule not. An easy method of looking for the most glaring .is ts follows: Unscrew the systems and examine each separately. Place one in the camera or on the table in an upright position, and behind it at any distance a lamp or gas jet. Look for the focal point by means of a white paper, and get the eye in the same position, which can easily be done after a little practice. The lens will be found brilliantly illuminated, and the striae will stand out as dark shadows. When a few small knife edge threads show, they are of little moment; but when in a circle of light there is a wavy appearance, it is generally indicative of other striae which are injurious, but cannot be determined in this manner.

Definition, distortion, and amount of speed can only be judged by a comparative test or by a person of wide experience. Comparing a lens with one of known excellence is the only reliable manner, but then the following points should be observed. Determine coincidence of visual and chemical foci. Determine coincidence of ground glass and plate in plateholder. Use flat plates when ground glass is flat; if this is not flat, use plates corresponding with it. Determine spherical correction by means of diaphragms.

The two lenses should be for the same size plate, and of about the same aperture and focus. Lenses should always be tested first with full aperture or large stops of the size, as small stops correct spherical aberration in good lenses as well as poor ones. If desired, they can be done with small stops later on. Make comparisons under the same conditions of light. For spherical aberration and distortion, focus on printed matter or on a map fastened to a screen, and have the lens square with the objective and within two or three times the equivalent focus of the lenses, as they are then under the most strained conditions. For depth of focus use stationary objects which have thickness, or a view. Always focus the lens on the same point, and never make a comparison without the use of a magnifying glass for focusing as well as for examining the negatives. Last, but most important, make the comparison in the spirit of arriving at the truth, and do not permit your judgment to be biased or influenced by your feelings.