In the mechanics of making a slide from a negative we have only two methods to consider. If the slide-image is to be the same size as the negative, or a part of the negative, we print by contact, that is, we put the negative and the slide-plate face to face in contact, and we expose to light, the negative being next to the light; this corresponds with making a print on paper. But when we desire to make a slide including all the subject of a negative larger than a slide-plate, or, in fact, when we desire to alter the size of the image at all, we copy the negative "in the camera." The simplest method of doing this is to fix up the negative so that it is evenly lighted and make a photograph of it in a camera; but the adjustments necessary for such an operation would be found awkward, and so a "reducing camera" of some kind is generally used. Many such cameras are on the market, and consist of devices for holding the negative in a suitable position with regard to a small camera furnished with a lens and a dark slide holding a lantern plate. Either the negative-holder or the camera should have possible movement in all directions vertical to the optical axis of the whole, and in addition it is often desirable to have a certain amount of movement in other planes, in order to correct certain optical defects that are sometimes found in negatives. The writer has for many years used a small camera with its front stuck into the front of a large camera, one or other of these cameras has every necessary movement. The device is figured here.
Whatever arrangement is used the end of the apparatus bearing the negative is directed towards a good and even light; and it is well to place about two inches in front of the negative towards the light a piece of finely ground glass for ordinary negatives; this glass is with advantage omitted with extra dense negatives.
It goes without saying that the exposure, whether we are working by contact or in the camera is of the utmost importance; but it is not possible in an article such as this to give even an approximate idea of the exposure suitable under any concatenation of conditions. The best clue to exposure is to be found in development, and in results. It is necessary to know what happens after normal exposure with a given developer, and then if any variation is noticed to alter the exposure. If a plate develops more rapidly than the normal, it may fairly be deduced that the exposure has been too long; but if we are dealing with a specially contrasted negative it is better so. On the other hand, when we are dealing with a thin negative, especially if the scale of gradation is short, we require an exposure less than what would under normal conditions lead to complete development in the normal time. And again some plates require to be developed to a greater point of apparent density than others; this is a matter of experience. Briefly put, there is no royal road to good slide-making, experience is necessary.
In actually making the exposures certain points must be kept in mind. If we are copying in the camera with daylight as illuminant it is very important to use the light from the north; if we use other light we shall be much put out on most occasions by awkward variations of the brightness. A very large number of operations are rendered nugatory by carelessness in this matter. Even the most experienced worker will find it impossible to expose plates with anything like accuracy when he has to deal with direct sunshine at one time, thin white clouds at another, and dark clouds at a third. And in making exposures by contact the beginner must be fairly accurate in judging the time of exposure and the distance from the radiant. A good plan is to tie to the gas jet a piece of cord having knots at convenient intervals, such as at 9, 12, 18 and 24 inches, and in making an exposure to use these knots as guides to the distance; moreover, the law of "squares of distance" must be remembered; the intensity of light varies inversely as the square of distance from radiant to receiver, provided, of course, no optical arrangement is introduced to modify the path of the rays. Consequently, for example, if ten seconds is found to be a proper exposure at nine inches from the light, the corresponding exposure at eighteen inches will not be twenty, but forty seconds (92=81. 182=324). And as it is sometimes awkward to hold the frame and attend to a watch at the same time, a metronome, ticking seconds, will be found convenient, or a clock with a second hand may be placed where it can be seen during the exposure. A landscape slide without clouds, if the horizon is in the picture, is usually considered a failure, and has been dubbed "bald-headed." Really good workers often put clouds into slides by "combination printing," which in some cases is comparatively easy, in others very difficult. If we are working by reduction from a good-sized negative, with a fairly even horizon, the difficulty is not great. The landscape part of the negative is first exposed, the sky being masked if necessary, and a cloud negative is then substituted for the landscape negative, a part of the former being masked to correspond with the landscape on the latter, and a second exposure is made on the same slide-plate. Admittedly in all cases this requires "some doing," in many cases it is extremely difficult. An easier, if less "sportsmanlike," method is to make the cloud slide on a separate slide-plate, and to use the latter as a "cover-glass" for the slide. The cloud may cover the whole of the second slide, and that part of it not required may be wiped out by means of a reducing solution, used with a brush, such as the ferricyanide and hypo one described later. This method will be found useful even in contact slide making, but it also requires not only good taste in the selection of the cloud, but some deftness in manipulation; but the neat-handed beginner need not fear to make the attempt.
DEVELOPMENT.
It has already been stated that judgment of exposure is a matter of experience, and that results are the best criterion; here follow some more explicit statements on the same matter.
Whatever plate or developer is used, and whatever the time occupied in complete development:
1st.—If by the time the high-lights are sufficiently strong the shadows are too dense or blocked, the plate has been under-exposed.