The colours reflected by the film are due to interference : they are of the same kind as those reflected by soap bubbles or by N ew ton’s rings. W hen a ray of definite w ave-length falls on the sensitive plate, it is during exposure reflected back by the m irror, and then gives rise to a set of standing waves in th e interior of the film, the distance between two successive loops being equal to half the wave-length of the lum inous ray. This system of standing waves impresses its periodical structure on the film. The photographic deposit, therefore, takes th e form of a grating, a continuous grating, perfectly adapted for reflecting the particular lum inous ray which has given it birth.
This theory can be subjected to experimental proof. I f we examine a photograph of the spectrum , or any other object by white light, we observe the following facts. (1.) Colours are seen in the direction of specular reflection, and are invisible in every other direction. (2.) The colours change w ith the incidence; the red changing successively to green, blue, and violet, when the incidence grows more oblique. The whole image of the spectrum is displaced, and gradually passes into the infra-red region. (3.) If the film be gradually moistened, the colour changes in the opposite direction, from violet to red. This phenomenon is due to the swelling up of the gelatine or album en, causing the intervals between the elements of the g rating to become larger. The sm aller intervals, corresponding to violet and blue light, gradually swell up to the values proper to red and infra-red waves. A photograph immersed in w ater loses all its colours, these appearing again during the process of drying. Tor the same reason, a freshly prepared plate has to be dried before the correct colours can be finally seen.
We have now to consider the case of compound colours, and to generalise the form er theory, which is only applicable to the action or simple rays. I beg to subjoin an abstract of this generalised theory. I t will be seen th a t if a compound ray of definite composition impresses the plate, it gives rise during exposure to a definite set of standing waves, w hich im press their stru ctu re on the film, and im part to the photographic deposit a corresponding definite form. Though very complex, this can be described as made up of a num ber of elem entary gratings, each corresponding to one of the sim ple rays which contribute the im pressing light. W hen examined by white light, the reflected ray is shown to have the same composition as the im piessing ray , 'white light, for instance, im parts to the photographic deposit such a structure th a t it is adapted to reflect w hite light.
The only a priori condition for the correct rendering of compound rays, is a correct isochromatisation of the film. This, again, can be practically effected by known processes, such as have been indicated by E. Becquerel, Vogel, Captain Abney, and others.
