The exposure in this, as in all photography, is the crux. Unless a meter is employed, there will be a great waste of plates. In a good light it is safe to assume that the Autochrome plate, with its yellow screen, has a speed of 2 Watkins, or of f/14 Wynne. In winter, and generally where the light is poor, say, wherever the exposure with f/8 according to the meter would be more than a minute, the exposure must be considerably increased. It may safely be doubled at the least. Watkins supplies a special dial for his meter to make this correction. The writer has not found any necessity for the increase when the light is good, and the exposure is merely made a long one by using a small stop.
As very little light is permissible for development, it is well to clear the bench of everything except what is wanted for the process. These are a dish and a card to cover it, a watch or dark-room clock, and two graduated measures. Into one measure is poured the developer and into the other a similar quantity of the " C," reversing, solution. The formula for this bath will be found with Messrs. Lumiere's instructions at the end of the article.
The time of development for a correctly exposed plate is said to be 2 1/2 minutes, but this is influenced by the temperature of the developer. In my own practice, I use a " Watkins' Thermometer for Autochromes." This is put into the developer for half a minute and indicates directly the time during which development should be continued. The dish can be kept covered all the time ; but there is no harm in glancing at the plate after it has been in the developer for at least a minute, if the dark-room lantern is provided with a " safe light."
Messrs. Lumiere have given instructions for the control of development, which are recapitulated at the end of this article ; but these have certainly not given me, at any time, results comparable with those that are to be got by correct exposure followed by development for a standard time.
After a few plates have been developed the photographer will find that he can form a very fair idea of the completion of the operation by the appearance of the surface of the plate as it lies in the dish. Nothing is gained by looking through it, and it must not be picked up for the purpose. If the surface is creamy white, with only the very brightest parts of the subject showing as dark patches, the plate is under-exposed. If it is black all over, except perhaps in the deepest shadows, it is over-exposed. If it looks very much as an ordinary plate looks when it is under-exposed and fully developed, the exposure has been about correct. In the properly exposed, properly developed Autochrome, almost every part of the surface will have darkened to some extent, but only the very brightest parts will be perfectly black.
Although the treatment above described will not cause the plate to be perceptibly fogged, it must not be forgotten that it remains sensitive even to red light until after development. The total exposure to the lamp for examination purposes must not exceed three or four seconds, or the colours will be weakened. If the plate is held up to the light and looked through, there is risk of the colours being made too red.
After development the plate, still in the dark, is rinsed for half a minute, and is then flooded with the " C " solution and the dish rocked. After it has been in this bath for two minutes, it may be taken out into daylight and examined. If it is exposed to daylight sooner than this, there is a risk of transparent spots being formed. By this time the reversing solution will have done its work, the just developed negative image will have been dissolved away, leaving its counterpart, a positive image in creamy white silver bromide. On holding the plate up to the light at this stage, the colours will be seen, but they will not be as brilliant as they afterwards become. When the " C " solution appears to have no further effect upon the plate, which will be the case in less than three minutes after its application, it is poured away, the plate is rinsed for a few seconds, and is then flooded with an ordinary amidol developer. A convenient strength is
2 1/2 grains.
This should be freshly mixed. If quinomet is used for the first development, the solution after use may be kept and used for this second development, instead of the amidol, but the pyro-ammonia developer is not satisfactory for this purpose. This second development can be performed very effectively with rodinal 1 part, water 20 parts.