12 On Colour Photography by the Interferential Method. 



The colours reflected by the film are due to interference : they are 

 of the same kind as those reflected by soap bubbles or by Newton's 

 rings. When a ray of definite wave-length falls on the sensitive 

 plate, it is during exposure reflected back by the mirror, and then 

 gives rise to a set of standing waves in the interior of the film, 

 the distance between two successive loops being equal to half the 

 wave-length of the luminous ray. This system of standing waves 

 impresses its periodical structure on the film. The photographic 

 deposit, therefore, takes the form of a grating, a continuous grating, 

 perfectly adapted for reflecting the particular luminous ray which has 

 given it birth. 



This theory can be subjected to experimental proof. If we ex- 

 amine 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 with the incidence; the red 

 changing successively to green, blue, and violet, when the incidence 

 grows more oblique. The whole image of the spectrum is dis- 

 placed, 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 albumen, causing the intervals between the 

 elements of the grating to become larger. The smaller intervals, corre- 

 sponding to violet and blue light, gradually swell up to the values 

 proper to red and infra-red waves. A photograph immersed in water 

 loses all its colours, these appearing again during the process of drying. 

 For 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 former theory, which is only applicable to the action 

 of simple rays. I beg to subjoin an abstract of this generalised 

 theory. It will be seen that if a compound ray of definite composi- 

 tion impresses the plate, it gives rise during exposure to a definite 

 set of standing waves, which impress their structure on the film, and 

 impart to the photographic deposit a corresponding definite form. 

 Though very complex, this can be described as made up of a number 

 of elementary gratings, each corresponding to one of the simple rays 

 which contribute the impressing light. When examined by white 

 light, the reflected ray is shown to have the same composition as the 

 impressing ray ; white light, for instance, imparts to the photographic 

 deposit such a structure that it is adapted to reflect white 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. 



