1876.] The Constants of Colour . 463 
blend into a grey. This grey can be altered in its brightness, 
till it seems about as luminous as the red. If we find, for 
example, that with the disc three-quarters black and one- 
quarter white, an equality appears to be established, we 
conclude that the luminosity of our red surface is 25 per cent 
Fig. 5. 
Facsimile of Rutherford’s Drawing of Six-Prism Spectroscope .— [American 
Journal of Science and Arts , 1865.) 
of that of white paper. This of course based on the suppo- 
sition that the black paper reflects no light ; it actually 
refledtsfrom 2 to 5 per cent, the refledting power of white 
paperj being put at 100. The results thus obtained are 
always inexadt, and the same observer will often obtain 
