Luminosity of Coloured Surfaces used for Colour Discs. 121 



Yellow. 



The luminosity of 118 = ^ of 78 = 25 -6 



ooU 



Green. 

 71 - of 43 - 8-5 









White. 

 130 = of 100 = 36-1. 



Blue. 

 The luminosity of 171 is therefore represented by 



36-1 -(25-6 + 8-5) = 2. 

 The luminosity of the blue pigment is therefore 



of 2 = 4-2. 

 171 



The luminosities of the three pigments were then compared with 

 white by the method described in Part II of " Colour Photometry," 



and found to be 







Yellow .......................... 77-7 



Green ........................... 43'2 



Blue .............................. 4-1 



The luminosity of the blue only differs by that found by the new 

 plan by (H, which is a very close approximation. 



The red disc was then interlaced with the blue and the green, and a 

 grey formed as before, and from calculation it was found that it had a 

 luminosity of 32 '5. Measuring it by the old plan, the luminosity 

 came out as 32'7. 



Having obtained the luminosity of the three standard colours, that 

 of any other colour can be calculated by substituting for one of them 

 a disc of such colour, and again making a grey and matching it with 

 a grey formed by the black and white. It will be noticed that this 

 method can be carried out in any light, whether candle light, electric 

 light, or day light ; but of course the luminosities of the colours will 

 vary according to the quality and kind of light employed. 



When the luminosities of the colours are determined, the angles 

 which the segments of the annuluses in fig. 1 should subtend can be 

 calculated after taking into account the luminosity of the black em- 

 ployed. 



When the disc is rotated round S, each colour should be equally 

 luminous, and if by means of an appropriate screen, placed in front of 

 the lens, the image of the disc impresses the photographic plate in such 



