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PHOTOGRAPHY IN NATURAL COLOURS 237
is the best sensitizer to use in this work, since it imparts a practically uniform sensitivity extending from deep red to blue and violet .
There are no gaps or maxima in the sensitivity curve of this substance, although it gradually increases in strength towards the violet. Absorbing solutions of wool black, cobalt sulphocyanate, and iron sulphocyanate reduce the action in the blue, green and yellow to that in the deep red, and by this means very satisfactory isochromatic action is obtained from red to ultra-violet.
In the actual photography of coloured objects fairly coarse-grained isochromatic emulsion should therefore be used. Ives found that a film about -^ mm. in thickness will give very good colours, but success depends very largely upon correct exposure and development ; very little deviation will render the colours weak. No exact data can be given as to time of exposure, experience alone must be the guide ; thus, working at //3*6 on sunlit objects, it is necessary to give exposures ranging from l\ to 5 minutes. Under the very best conditions as to light and with a lens working at //6, we must therefore allow at least 4 minutes for an exposure.
When the plate has been developed and fixed in dilute hypo-sulphite of soda, in which it is not allowed to remain too long, it is next necessary to arrange it so that it can be viewed by light reflected from the silver particles in the film, for when viewed in the ordinary way no colours can be seen. Now, in the photograph of any coloured object there will be a few layers of laminae, behind which will come a diffuse deposit, so that arrangements must be made to exclude all light except that coming in the direction to be regularly reflected by the lamina?, since if fight reflected from the diffuse deposit is also obtained, the colours will be very much weakened, for they will then be mixed with a certain amount of white light. If the film be made excessively thin, there is, of course, the ad