THE LEAST VISIBLE TINT AND THE FIRST APPEARANCE TIME The least visible tint is the faintest spot which it is possible to distinguish on any tinting med ium on exposing it to light behind an opaque screen in which a hole has been cut (p. 35). The tint is observed by raising the hinged corner of the cover away from the tinting strip when, if the exposure has been just right, the form of the opening will be just visible in contrast with the original white or light color of the same medium where it was protected from the light.
The advantage of the least visible tint as a standard is that no device is necessary on which to carry another tint for matching, as the com parison is made, as has been said, between the tint and the untinted part of the same medium. Also the method of comparing different tinting mediums which will be given (pp. 47, 156) enables any desired medium to be used in con nection with its own tinting speed factor referred to a chosen medium as a standard.
The time required under any circumstances to obtain the least visible tint is termed the first appearance time, or in practice simply the "tint time" of the light. If it be the time as taken with the standard or f / 1 meter it may be spoken of as the meter time.
Every photographic worker should under stand perfectly how to secure the least visible tint. A feeling of doubt generally assails one on first attempting to obtain it, but a little prac tice under various conditions of light will over come this feeling entirely.
One should become familiar with what may be termed the disappearing scale of tints which will now be explained. For experimenting use strips of gelatine p.o.p. such as "solio" already
mentioned. Having placed one of these strips behind the contact opening in the back of the book, first having covered the opening with a coin as has been directed, make a series of exposures under the same conditions except that each is half as long as the preceding. Stand a few feet back from a window and start with an exposure of 32 seconds, having the book face the window. The strip should be shifted to a fresh place for each successive tinting. On examining the series of tinted spots the first will no doubt show considerable discoloration while the depth of tint decreases with a characteristic gradation which will at once be recognized. If the experiment be successfully performed there will be one exposure which results in a tint only just distinguishable from the un tinted part of the paper. This is what has been termed the least visible tint and the one intended by the author to be the tint sought for in the measurement of light by the present system. The time required to make this tint is the first appearance time or the tint time of the light as already explained. The accompanying figure illustrates the characteristic gradation of the disappearing scale of tints.
After a few experiments in making this dis appearing scale one becomes so familiar with the gradation steps as to be able to estimate at a glance, on securing any tint, its approximation in the scale so that a single other trial will usually result in the desired least visible tint.