356 SCIENCE PROGRESS 



these films has been published quite recently by Prof. S. R. 

 Cajal of Madrid. As the resolution of the laminae requires 

 an objective of a numerical aperture equal to or greater than 

 the greatest available, Cajal swelled his sections in water and 

 so obtained excellent definition. He has photographed sections 

 of all kinds, and analysed the effects by such methods as 

 rubbing away the surface to distinguish between the effects 

 of the superficial and the underlying parts. He confirms 

 Zenker's theory exactly, except, perhaps, with regard to white. 

 This he found was not due to the production and mixing of 

 all colours or even complementary colours, but to simple 

 reflection from a superficial deposit of silver ; and in tinted 

 whites or light colours he also found this superficial layer, 

 but here it is partly transparent, and the interference colours 

 produced by the laminae below show through it. 



Although attempts have been made, up to quite recently, 

 to perfect the methods in which silver subchloride is the 

 sensitive material, Lippmann's process may be regarded as 

 the final result of all attempts to photograph colour directly. 

 It is an exceedingly beautiful process to the experimentalist, 

 but it has so many possibilities of error that it is very un- 

 certain, and therefore unsuitable for general purposes as a 

 method of photography in colours. 



II 



The Young-Helmholtz theory of colour vision supposes 

 that the normal human eye is sensitive to only three funda- 

 mental colours ; that is, that the points of sensitiveness of the 

 retina, whatever they may be, are of three kinds. When white 

 light falls upon the retina all three are excited to a definite 

 (or equal) proportion, and when this proportion of excitation 

 varies we become conscious of colour. Granting this, it follows 

 that if the colour can be found that corresponds to each kind 

 of sensitiveness, we shall have three colours that will, by their 

 mixture, serve to imitate every possible colour, including, of 

 course, black and white. These three fundamental colours have 

 been determined from time to time by various investigators, 

 and may be roughly described as red, green, and blue. We 

 may then, for reproduction purposes, regard the light reflected 

 from any object whatever, when illuminated in any manner 



