802 Captain Ahney [Fob. 25. 



WEEKLY EVENING MEETING, 



Friday, February 25, 1893. 



Sir Frederick Abel, Bart. K.C.B. D.C.L. LL.D. F.R.S. 

 Vice-President, in the Chair. 



Captain Abney, C.B. D.C.L. F.R.S. 3IM.L 



The Theory of Colour Vision applied to Modern Colour Photography, 



The subject of my address this evening is a very large one, and 

 would occupy moie time than the hour allotted to me, if I entered 

 fully into every part of it. All I can hope to do is to jiut before you 

 the main scientific reasoning which has led to the success at present 

 attained in colour photograj)hy, by a combination of colours, aud 

 by the absorption of colouring matter. 



On the screen we have the sjiectrum of the electric light, and a 

 very beautiful object it is. But it is not to its beauty that 1 wish to 

 call your attentiou, but to the varying brightness of its different 

 parts, and further, to the fact that in it we have strictly pure colours, 

 that is a series of simple colours, and not mixed colours such as we 

 may find in nature. Now if we can reproduce fairly well by means 

 of photography this grand multi-coloured band, both as regards 

 colour and also brightness (that is luminosity), we may say that we 

 have succeeded in doing what is required, and that all hues in nature, 

 with their varying shades and brightness, can be equally well repro- 

 duced. The exponent of colour photography is bound to go to the 

 spectrum for his information, and this I must do to-night. On the 

 wall is a diagram of the spectrum in the shape of a curve, which 

 shows the luminosity of every individual part. If we could abolish 

 colour from our minds, and merely look upon the spectrum as a 

 monochromatic band having waves of difierent oscillation frequency, 

 we should have this same curve, and our eyes would be like a photo- 

 graphic plate, which knows no colour qua colour. All that the plate 

 knows is that a certain wave length, having a certain amplitude, will 

 so afi*ect its sensitive surface that a certain opacity of deposit will 

 be attained on applying the developer to it. If two or more colours 

 are mixed, each of the wave lengths will play its own part, and an 

 opacity will be produced representing the sum of the separate effects. 

 A little reflection will show that whatever photographs we may obtain 

 we must use outside coloured light to illuminate them if a coloured 

 object is to b^ reproduced. V7e have to consider what are the fewest 



