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VIII. Ilic Colour Sensations in Terms of Luminosity. 

 By Captain W. DE W. ABNEY, C.B., D.C.L., F.R.S. 



Received February 23, Read June 15, 1899. 



(I.) Introductory. 



MY attention has been directed recently to the theoretical considerations involved in 

 the production of photographs in approximately the colours of Nature, by combining 

 together the images from three positives backed by appropriate colour screens, the 

 colours chosen being those which should best represent the three Colour Sensations of 

 the Young Theory. 



1 Miring my investigations into the matter I found it necessary to ascertain what 

 these colours were, for although serious objections may be raised to the Young Theory 

 when considering it in detail, yet when expressed in a general form it adequately 

 explains the phenomena which arise when colours fall upon the centre of the 

 retina. The sensation curves have been given by KCENIG, but it appeared that a 

 redetermination by a luminosity method might well be undertaken, for they did not 

 altogether agree with the results of some preliminary measures that I had made in 

 order to trace them. In my work on ' Colour Vision' I have given a rough diagram as 

 to what the sensation curves might be when they are shown as luminosities which 

 together make up the total luminosity of the spectrum of the crater of the arc light, 

 but it was only intended to be an approximation to the correct diagram. The 

 method, however, by which the problem could be attacked and by which a rigid 

 determination could be made was indicated. It is by this method that the results 

 given in the following pages have been obtained. 



(II.) A Preliminary Survey. 



The red sensation can be perceived in purity at one end of the spectrum. From 

 the darkest red to a point near the C line, a little above the red lithium line, the 

 colour is the same, though, of course, the brightness varies, but the brighter red colour 

 can be reduced to form an exact match with the dark red, and no mixture of any 

 colours will give a red of the description we find at the end of the spectrum. 



At the violet end of the spectrum we also find that the colour is the same throughout, 

 In mi the extreme visible limit to a point not far removed from G, but it is not for 



2L2 11.12.99. 



