156 Colour Fhotography. [April 17, 



iodide to half per cent, of the albumen, we get a perfectly transparent 

 plate, adapted to colour photography. 



We want now to go a step further. It is very well for physicists 

 to be contented with working on the spectrum, since that contains 

 the elements of every compound colour ; but we all desire to be able 

 to photograph other objects than the spectrum — common objects with 

 the most compound colours. We have again but to take theory as a 

 guide, and that tells us that the same process is able to give us either 

 simple or compound colours. We have then to take a transparent 

 and correctly isochromatised film, expose it with its mercury backing, 

 then develop and fix it in the usual way; the plate, after drying, 

 gives a correct coloured image of the objects placed before the camera. 

 Only one exposure, only one operation is necessary for getting an 

 image with every colour complete. 



A plausible objection was offered at first to the possibility of 

 photographing a mixture of simple coloui-s. The objection was this : 

 a ray of violet gives rise to a set of strata separated by a given 

 interval ; red light produces another set of strata with another 

 interval ; if both co-exist, the strata formed by the red are sure to 

 block out here and there the intervals left between the strata formed 

 by the violet. Is it not to be feared that one fabric will be blurred 

 out by the other, and the whole effect marred ? The confusion would 

 be still worse if we consider the action of white light, which contains 

 an infinity of simple components ; every interval here is sure to be 

 blocked up. 



Mathematical analysis, however, shows this objection to be 

 unfounded ; we have great complexity, but not confusion. Every 

 compound ray, both coloured and white, is faithfully rendered. As 

 an experimental proof of this, we will project on the screen photographs 

 of very different objects, namely, stained glass windows, landscapes 

 from nature, a portrait made from life, and vases and flowers. 



That the colours here observed are due to interference, and not to 

 the 23resence of pigments, can be shown in the same way as with the 

 spectrum. Here, again, we observe that the colours are visible only in 

 the direction of sj^ecular reflection, that they change with the angle 

 of incidence, that they change and disaj)pear by wetting, and reapj)ear 

 by drying. Pigments remain equally visible and unaltered in colour 

 under every incidence. If we attempted to touch up one of our 

 photographs with oil or water-colours, the adulterated place would 

 stand out on a colourless background by merely observing by diffused 

 light. It is therefore impossible either to imitate or touch up a colour 

 photograph made by the above-described interferential method. 



[G. L.] 



