AND ITS APPLICATION TO THE DETERMINATION OF COLOUR SENSATIONS. 355 



(24.) Practical Application of the above Principle. 



The method shown of ascertaining the composition of the colours in terms of the 

 three sensations, and of ascertaining their luminosity, enables us to make an accurate 

 determination of the amount of reduction which the various pigments should undergo. 

 Suppose we wish, for instance, to make the red sensations in the yellow, the green, 

 and the white the same, we should proceed as follows : 



The amount of red, green, and blue sensations in these three on the same empiric 

 scale are 



If we reduce these sensations to colours, from Table IX. then we shall have for 

 (say) the red component in white 342, in chrome-:yellow 284, and in emerald-green 79. 



In order to reduce all these to show equal red components, the centre of the disc 

 would be occupied by emerald-green pigment. The chrome-yellow would have to 

 be reduced to - 2 \ a 4 -, or '278 of its normal luminosity, so that '278 of 360, or 260 of 

 the annulus, would have to be occupied by dead black. 



The white would have to be reduced to -g 7 ^, or '231 of its normal luminosity, so 

 that 277 of the annulus would have to be occupied by dead black. 



If a green screen had to be obtained the green sensations reduced to green colour 

 would be white 447, chrome-yellow 298, emerald-green 255. Then, as before, 

 emerald-green would occupy the centre of the disc, and chrome-yellow would have 

 to be reduced to fff, or '856 of its luminosity, and white to f I*, or '534 of its 

 luminosity. 



The above will give an idea of the method to be adopted in making what I have 

 called colour sensitometers. Examples have been given only with those pigments 

 which have been considered in the foregoing pages ; but naturally there would be 

 many other colours introduced in order, as far as possible, to imitate the spectrum 

 colours. 



2 z 2 



