2nd.—If by the time the high-lights are sufficiently strong the shadows have not attained sufficient density, or are veiled, the plate has been over-exposed.
3rd.—If at the same moment the high-lights show sufficiently and the shadows are transparent but sufficiently plucky, the plate has been properly exposed.
4th.—With a normal negative the normal exposure is the proper exposure; but (a) a negative abnormally strong in contrasts will require an abnormally long exposure; and (b) a thin negative, or one with a very short scale of gradation from densest to clearest, will require an abnormally short exposure, with probably some after-treatment in the direction of "intensification."
Ambitious slide-makers generally aim at warm-toned slides for pictorial effect, and rightly. But slides intended for scientific purposes are generally better when cold in tone, the definition is usually better. But whatever the aim, a good tone of one kind is preferable to a poor one of another kind, and the beginner should first make sure of getting a really good cold tone, which is comparatively easy, and then try his "'prentice hand" on warm tones.
Warm tones are obtained by using greatly super-normal exposures and greatly restrained developers; and the danger probably lies in the fact that the long exposures are apt to lead to fog, and the great restraint to over-density in the shadows, the latter especially when the exposure has not been quite long enough for the developer used; herein probably lies the whole secret of warm slide-making. If we aim at really warm tones and use developers suited to such design, we must on no account stint the exposure.
COLD TONE DEVELOPING SOLUTIONS.
The classical solution for cold tone slides is a solution of ferrous oxalate in potassic oxalate. Of all developers it is most free from fogging propensities. It is made from so-called "saturated solutions" of proto-sulphate of iron and potassium oxalate. Thus, into a bottle put a quantity of iron proto-sulphate, and pour on about three times its weight of water containing a dram of sulphuric acid to each pint. Shake well, and keep always at about 60° Fahr.; some of the iron must always be visible in the bottle, if not, more is to be added. The crystals of iron salt must be green and not rusty in colour. This is the "iron solution."
The "oxalate solution" is made by dissolving potassium oxalate in about three times its weight of water. This also must be kept at 60° Fahr., shaken occasionally, and oxalate added if none is visible in the bottle.