The next step is cementing. This is a delicate process, for although in each part of the work heat has been used, a much higher degree is required, and the danger of cracking the lenses is increased. It is no ordinary occurrence to lose lenses in this work, or, for that matter, at. any point of the process. Great care must here be observed to prevent the cement from turning yellow, thus disturbing the transparency of the lens and causing loss of light. The lenses are next temporarily fixed in the mounting, and are ready to be examined in the camera. Now cornea the moment of suspense. Focusing an an object - and for this purpose a large flat screen covered with printed matter containing various sizes of letters, or a large map hung against a perpendicular wall, is the best - the first thing which comes to our notice is the focal length of the objective. It may be one-half of that which was figured upon, or twice as long. However, if not correct, we make note of how much it varies, and proceed to examine the image.

This may be sharp and brilliant in the centre, and gradually become more indistinct as it nears the edge, and may remain indistinct near the edges under any amount of focusing; or it may have the same fading out of the image toward the edge when the central portion is sharp, but this may be brought out clear by change of focus; or the image may be such that it cannot be sharply focused at any point. There is yet one possibility, but a strong improbability, that the image may be quite sharply defined over the entire screen; this of course is the most favourable condition.

After making note of whatever the appearance may be, we proceed to determine the coincidence of visual and chemical foci. As is well known, there are, photographically speaking, two sets of light rays emitted by every object. Those that form the image on the screen, and which are most perceptible, or the visual. ones, are made up by the red, yellow, and orange colours of the spectrum; whereas those which make the photographed image, or the chemical rays, are formed by blue, green, and indigo, which are the least discernible, and it is a problem of some difficulty to bring both sets to form an image in one plane. To determine this, we proceed to make an exposure on a plate. Before doing this, however, we must be assured of three things: First, that the ground glass is flat; second, that the plate which we intend to use is flat; third, that the plate when exposed will be in exactly the same plane as the ground glass. Upon these points depends a reliable result, and with many plates and cameras these conditions are not fulfilled.

If you will go through a package of plates you will find some almost perfectly flat, others that are convex, and others concave, the deviation being fully as much as 1/16 in., and in larger plates even more.

Ordinarily, and particularly when stops are used, it is of little moment, and it would be almost impossible to furnish selected flat plates at the present prices. The Eastman roll holders are well adapted, because the film passes over a fixed and flat back which remains constant in its relation to the ground glass. If the ground glass is flat, then flat plates should be selected, and if the ground glass be concave, then plates corresponding to it should be used. As a rule, where care is observed, slightly concave ground glass plates are used, because this form favours the lenses. The one thing, however, which, should be insisted upon, and which is not always, the case, is that the film side of the plate should be exactly in the plane of the ground glass when it replaces it. To determine this the following plan is effective.

Take a straightedge and allow it to rest upon the face of the frame of the ground glass which rests against the back of the camera. Cut a small wedge of wood, and insert this between the straight-edge and ground glass as far as it will go, and mark the edge in line with the straight-edge, when it fills up the space. Now take a plate which is suited to the plate holder and which is known to be flat, and place it in this, and by the use of the straight-edge and wedge proceed as above with the ground glass. It is unnecessary to say that the marks should agree exactly, and if not, should be made to do so.

These precautions are alike necessary for the optician and photographer, whether for the purpose of determining the quality of the leas, or for making comparisions between several lenses, and it is a test which every camera ought to undergo. After the plate has been exposed and developed, we may find that although we have a sharply defined image on the ground glass, the one on the plate is very indistinct, which shows that the chemical foci may be shorter or longer than the visual. To determine the extent of variation a number of exposures must be made, placing the ground glass within or beyond the visual focus until as sharp an image is obtained in the negative as on the ground glass. We now have all the principal data to enable us to proceed with the correction of the lens. First is the focal length, next is the spherical aberration, then the amount of distortion, and last, but for the present most important, the difference between visual and the chemical focus.

To attempt to correct any one of these qualities will disturb the others, and not only that, they are often to such an extent antagonistic that to improve one will considerably augment another fault. For instance, if the visual and chemical foci do not coincide, it is evident that the objective is chromatically over or under corrected, and if the objective would otherwise give a good image, the inner or cemented surface would probably be changed; this might very naturally affect the focal length, which in a telescope objective would hardly be noticed. The focus of the lens might easily be doubled before the proper correction Would be reached, and then changes would again be necessary to bring the focus to the proper distance, which would again disturb the chromatic correction as well as perhaps the spherical, which latter designates the amount of curvature of the image. Thus a change of 1/16 in. in the radius of one of the inner surfaces, which would hardly be noticed in an ordinary lens, might lengthen or shorten the focus of the photographic objective several inches. The changes which sometimes occur are often surprising and inexplicable, even to one who is accustomed to them.