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X. Modified Apparatus for the Measurement of Colour and if* Application to 

 tin- Determination of the Colour Sensations. 



Ht, Sn- WILUAM I.K W. ABXEY, K.C.K., f'.It.S. 



Received April 17, Head May 18, 1905. 



PART 1. 

 (I.) Introductory. 



IN a paper contributed to the ' Phil. Trans.' in 1.899 on Colour Vision, the colour 

 sensations in terms of luminosity were given in detail. Since that date a large part 

 of the leisure which 1 could command outside my official duties has been occupied in 

 revising the measures there given. To effect this revision, a modification of the 

 apparatus I previously employed was carried out. Some slight alteration in the 

 sensation curves was the result, and. though small, ought to be recorded. 



The principal alteration that lias to be made is in the amount of what may be 

 called "inherent white" which exists in the spectrum colours. The white is due, 

 at all events in part, to the overlapping of the three sensations. It will in the first 

 instance lie necessary to describe the change that has been made in the colour-patch 

 apparatus with which my previous measures had been made, since a good deal of the 

 alteration in the blue sensation curve between X. 5000 and X 5100 is dependent on it. 



(2.) The Colour-patch Apparatus. 



The colour-patch apparatus is now arranged to enable two spectra formed by the 

 same source of light to be used either separately or together. This arrangement 

 allows a comparison of any differing mixtures of spectrum colours to be made, and 

 it also allows the addition of any desired quantity of white light to the colours 

 formed by the aid of either of the two spectra, 



In the original apparatus the intensity of the white light used for comparison with 

 the colours varied with the intensity of the spectrum. The mode adopted to secure 

 this result was to use the light reflected from the first surface of the first of the two 

 prisms used in forming the spectrum. The beam of white light so obtained was 

 reflected by a mirror on to the screen, on which the patch of colour was thrown. 

 In the modified apparatus this principle of reflection has been still further utilized. 

 The white light is used as before to form the spectrum to the comparison light, but, 

 in addition, the light, after passing through the two prisms, passes through a half- 



VOL. ccv. A 396. 17.11.05 



