In order to obtain by this process the repro duction in colour of any subject, the Autochrome (of which we assume for the moment that the emulsion is capable of darkening equally under the three kinds of grains illuminated by white light) must be exposed in the camera with its back (bare surface of the glass or film) turned towards the lens, so that the light from the lens does not reach the emulsion until it has passed through the mosaic screen formed of the coloured starch grains.
The light from a yellow object is absorbed by the blue grains, but passes freely through the green and red grains, bringing into developable condition the crystals of silver bromide behind these grains. Similarly, the light from a red object is absorbed by the blue and green grains, but can pass the red grains, and bring the emul sion into developable condition behind such red grains.
Under these conditions, if the plate were developed and then fixed, a yellow object would be rendered as blue-violet, a red object by a greenish-blue, which is a mixture of the lights transmitted by the blue-violet and green grains, the image being thus a negative not only in respect of its luminosities (a black being rendered as white, and inversely), hut also as regards its colours.
If, after development, the image is reversed ("§ 442) by dissolving the reduced metallic silver and re-development of the residual silver bromide, a positive image is obtained in which each colour of the subject is correctly reproduced if the exposure and development have been correct, the colour at each point being comple mentary to that which would have been obtained if the plate had been fixed after development.
887. Compensating Filter. The ideal ortho chromatic quality that we have assumed to be possessed by the emulsion does not exist, and so it becomes necessary to com pensate for the differences in the spectral sensitiveness of this emulsion by a compensating filter used at the time of exposure. For work in average daylight, this filter must absorb the ultra-violet' and the extreme violet, and reduce the blue and green rays (except the blue-green corresponding with the minimum sensitiveness of the emulsion). These requirements are satis fled by an orange yellow filter, which is usually made by a suitably proportioned mixture of a yellow dye (absorbing the violet and reducing the blue) and by a pink dye (weakening the green).
When a compensating filter is used with a film with a tricolour mosaic of either irregular or regular pattern, it should preferably be placed in front of the lens. Except in the case of lenses of ultra large aperture, any adjustment of the focussing, due to the fact that the back surface is in the plane normally occupied by the emul sion surface, is generally unnecessary owing to the base being sufficiently thin to be negligible.
When using plates with a tricolour mosaic, the necessity for exposing the plate through the glass causes a difference in the focus obtained for any other sensitive material, and this is par ticularly inconvenient with hand cameras in which focussing is done by scale. Fortunately, it is possible to compensate for the reversal of the plate by placing behind the lens (after focus sing as for an ordinary plate) a filter which is twice the thickness of the plate.
If, in the beam forming a sharp image at 1 (Fig. 219), we place a sheet of glass of thickness E and with parallel sides, the sharp image will be removed to I'. By placing a sheet of glass of thickness e and parallel sides in a position parallel to the first glass and such that (in the absence of the first) the sharp image I would be formed on its front surface, the sharp image will be removed from to I". If the two glasses have the same index of refraction I5, the condition for I" to be formed on the rear surface of the second glass is E 2e.
Various other methods have been devised for ensuring the automatic correction of focus in the few cases where the construction of the camera does not permit of easy access to the back of the lens for placing in position and removing the colour filters. Among these are special dark slides which bring the front of the plate about i mm. in front of the plane where the image would be formed in the absence of the plate, or the use of filters with surfaces worked so as to form a very slightly divergent system (E. Wandersleb, 1907)." 887a. The Use of Artificial Lights. As with three-colour work, Autochrome photography may be done with any artificial light of the same qualitative composition as daylight, i.e. including almost all the visible rays, their relative propor tions being immaterial.