1896.] on Colour Photographif, 153 



presence of a mirror during exposure causes the colourless deposit to 

 show bright colours. Of course we want to know how this is done ; 

 we require to understand the theory of those colours. 



We all know that colourless soap- water gives brilliant soap-bubbles ; 

 the iridescence of mother-o'-pearl takes birth in colourless carbonate 

 of lime ; the gorgeous hues of tropical birds are simply reflected 

 from the brownish substance which forms the feathers. Newton 

 discovered the theory of these phenomena, and subjected them to 

 measurement ; he invented for the purpose the experiment called by 

 the name of Newton's rings. Newton showed, as you know, that 

 when two parallel reflecting surfaces are separated by a very short 

 interval, and illumined by white light, they reflect only one of the 

 coloured rays which are the constituents of white light. If, for 

 instance, the interval between the reflecting surfaces is only t-qIq o of 

 a millimetre, violet rays are alone reflected, the rest being destroyed by 

 interference : that is, the two surfaces send back two reflected rays 

 whose vibrations interfere with one another, so as to destroy every 

 vibration except that which constitutes violet light. If the interval 

 between the reflecting surfaces be augmented to toIo^ millimetre, 

 the destruction of vibration takes places for every vibration except that 

 of red light, which alone remains visible in this case. 



If we consider now this photograph of the spectrum, and especially 

 the violet end of the image, we find that this is formed by a deposit 

 of brown reduced silver. In the case of an ordinary photograph, 

 this deposit would simply be a formless cloud of metallic particles ; 

 here the cloud has a definite, stratified form; it is divided into a 

 number of thin, equidistant strata, parallel to the surface of the plate, 

 and To§o 0- uiiHinietre apart These act as the reflecting surfaces 

 considered by Newton, and as they are at the proper distances for 

 reflecting violet rays, and these alone, they do reflect violet rays. 



The red extremity of the photograph is equally built up of strata 

 which act in a like manner ; only their distance intervals here amount 

 to tfSoo niillinietre, and that in the proper interval for reflecting 

 red light. The intermediate parts of the spectral image are built up 

 with intermediate values of the interval, and reflect the intermediate 

 parts of the spectrum. 



The appearance of colour is therefore due to the regular structure 

 above described, imprinted on the photographic deposit. The next 

 question is — How has this very fine, peculiar and adequate structure 

 been produced ? 



It is well known that a ray of light may be considered as a 

 regular train of waves propagated through the ether, in the same 

 way as waves on the surface of water. The distance between two 

 following waves is constant, and termed the wave-length ; each sort 

 of radiation, each colour of the spectrum, being characterised by a 

 particular value of the wave-length. Now when a ray of light falls on 

 a sensitive film, this train of waves simply rushes through the film 

 with a velocity of about 300,000 kilometres per second ; it impresses 



