Lantern Slides.
A lantern Slide is a transparent positive on glass or other transparent support, usually 3¼ inches square, and is placed in the lantern in such a way that by suitable illumination and optical arrangements the image on the slide is made to intercept some of the light given off by the illuminant from a screen, which without the screen would be wholly and evenly illuminated by the light.
In viewing a paper print we are really observing the paper by reflected light, part of our view being intercepted by the image formed of pigment or reduced metal; the amount of light not being very great a very thin layer of pigment is required to produce the appearance of a sufficiently robust image. If the image alone or with its vehicle be stripped from a good print on paper this image viewed by transmitted light will be found to be extremely faint, far too faint to be of any use as a "transparency," and also too thin to be of any use as a lantern slide. On the other hand, what we know as a "transparency," such as is often used for window decoration, backed, perhaps, with ground glass, would be found much too dense or robust for use as a lantern slide. In density, then, a "slide," as it shall hereafter be called, comes between the image on a paper print and that on a "transparency." In "gradation," or gamut of tones, the slide ought to be superior to either the paper print with its almost absolute clearness over large areas, or the transparency with its dense shadows and its comparatively heavy lights. In fact, in a good slide we have every grade of deposit from perfect transparency to nearly complete opacity. But the extremes must be very sparingly present, and the transition from one tone to another must be gradual, all intermediate notes between highest and lowest should be represented.
Other qualities go to make the perfect slide; the metallic or other deposit forming the image must be in the utmost degree fine, no approach to "grain" must be perceptible even under the highest magnification. The colour, or "tone," must be not only pleasant but appropriate.
The loss of light in its journey from the illuminant to our eyes is enormous; the disc on the screen is simply a greatly magnified image of the light, and here is great loss; add to this the interception of some light by the opacity of the slide, and the fact that much more is lost in reflection from the screen, and absorption by the screen, and it is easy to realize that the image from the screen reaching our eyes is vastly less bright than that reaching the eye when, for instance, we examine a slide in the hand by transmitted light. And loss of light means increase of contrast, so that a slide which would seem about right in gradation in the hand would be altogether soot-and-chalk as a screen-image. So too if we have in the slide already shut out much light, by making the slide foggy, or over dense, it is easy to see that when the image reaches our eyes from the screen this vicious opacity will be immensely increased in its mischievous properties. The first style of slide gives screen-images sometimes called "midsummer snow-scenes"; the other slide is simply called "foggy." Both must be assiduously avoided.
It need hardly be said that the plates prepared by some processes are more likely to yield good slides, such as are described above, than plates prepared by other processes; no one process can claim to possess in itself superiority in all respects. Collodion, for instance, is less apt to give foggy slides, and it is easier to intensify than gelatine, but it is also more prone to give "hard" images. Collodion is at its finest in the form of collodio-bromide emulsion, which gives slides remarkable for fineness of grain, for clearness, and for richness of tones; but when we have to copy in the camera, the operation with collodion emulsion is protracted, unless we have bright daylight or a condensing arrangement, which with large negatives is often out of the question. On the whole it may be taken that gelatine-bromide emulsion is the process to be recommended, not only on account of its convenience and celerity, but in view of the many inherent points of excellence that it possesses. In any case, want of space will cause us to confine attention here to that process, and any one mastering the use of gelatine-bromide slide-plates will have nothing to fear from competition with other processes in all-round work. The writer has a leaning towards slide-plates as slow as he can procure them, because slowness almost always goes hand in hand with fineness of grain and freedom from fog.
There is one point of importance that should be noted in working with gelatine for this purpose. Distilled water should be used if possible for all solutions. Tap water—especially hard water—is apt to produce with the gelatine a certain amount of scum which, if present in any appreciable degree, cannot fail to affect the quality of the slides; but treatment with an acid alum bath as described later has a very salutary effect in removing any scum that may have formed during the "liquid" operations.