By using with artificial light the same com pensating filter usually employed with daylight, a faithful reproduction will be obtained of the colours of the subject at the time the photograph was taken—a very different appearance from that of the same subject illuminated by daylight. A white surface illuminated by an incandescent lamp seems white, just as in daylight, but ap pears yellowish or orange if we are able to com pare it, at the same time, with an identical surface illuminated by daylight (or by artificial light which has been given the quantitative composition of daylight by use of a blue com pensating filter). At the same time, the blues become dark grey, the light greens become yellow, and carmine becomes vermilion. These falsifications of colour by uncorrected artificial lights are, of course, well It is possible to imagine the use of correcting filters serving to bring the composition of the artificial lights used to that of daylight, such a, filter being superposed on the normal compen sating filter when taking the photograph, or superposed on the picture when it is being viewed. When we remember that the normal compensating filter for Autochrome photography in daylight absorbs blue-violet chiefly, the lack of which in artificial light is compensated for by the correcting filters absorbing green and red, it is clear that the superposition of the two filters will absorb the major portion of all the spectral rays, thus greatly increasing exposure.
On the other hand, if a correcting filter is super posed on the completed Autochrome, its already poor transparency will be still further reduced. For these reasons, it is necessary to use a special filter for each kind of artificial light,' so that the light transmitted through that filter produces equal densities of the emulsion through the three kinds of grains of the mosaic screen.
According to the light with which they are to be employed, these compensating filters arc of a less orange-yellow than the normal filter, and sometimes even of a greenish-yellow. They are always very much less absorbent than the normal filter.
888. The Dufaycolor Process. The screen of the Dufaycolor films is formed of equidistant parallel red lines, 20 to the millimetre. Between them, green and blue-violet squares alternate at the rate of 20 squares of each colour per milli metre. This regular screen is obtained as follows : The base is first coated with a very thin layer of collodion dyed blue ; afterwards a set of greasy ink lines is printed at an angle to the length of the roll ; in the next operation the dye between the ink lines is bleached, and a second dye bath results in the clear space being coloured green. The ink lines are now removed and a second set printed at right angles to the first. A second bleaching bath now removes the
green and blue dyes where there is no protective cover of ink. In a final bath, these clear spaces are dyed red, and lastly the ink lines are re moved. In each of the two rulings the thickness of the lines is such as to equalize approximately the areas of blue, green, and red.
On top of the reseau, to protect it, there is a very thin layer (about of varnish, thus avoid ing any effect of parallax, and then a coating of a special very highly sensitive panchromatic emulsion. The varnish used forms an insulator and very effective substratum for the emulsion, which has no tendency to come away, even in baths of 75° F.
The individual elements of the screen arc cer tainly much larger than the grains of an irregu lar mosaic, but there is no massing of elements of one and the same colour, and the structure of the image becomes apparent only in the case of projection on a very much enlarged scale viewed too closely.
889. Exposure. As in all cases where a very thin coating of emulsion is used, and particularly where reversal is carried out, the exposure lies within very narrow limits.
Whereas in monochrome photography, where there is a fairly wide latitude in exposure, the exposure is usually timed for the shadows, since over-exposure of the high-lights is rarely a, source of failure, in Autochrome work, how ever, the exposure needs to be for the high-lights of the subject (the sky excepted, which must be shielded during exposure by one of the means mentioned in § 125, but not graduated yellow filter).
Photographs taken in dull weather show less strong colours than those taken in sunshine, and give a better rendering of aerial perspective, but the exposure must be proportionately longer when the subject is not so strongly lighted.
In using an actinonieter, allowance must be made for these peculiarities, especially as regards decreasing the indicated exposure when exposing in very bright light and in increasing it in bad light. When taking ill-lit interior views it is often necessary to double the exposure calcu lated on the basis of the exposure necessary for a sunlit landscape.
890. Display and Projection of Colour Screen Plates and Films. Plates and films intended for direct inspection must be placed in passe-par tout mounts with wide, dark margins to mask the surroundings, which by their much greater brightness would cause the image to appear very dark.
The dyes used for staining the elements of the mosaic or of the reseau cannot stand the action of direct sunlight indefinitely. It is there fore necessary to avoid exposing colour plates or films at a window, except one facing the north.