462 
The Constants of Colour . 
[October, 
These measurements were made on a spedtrum obtained by 
a glass prism, which, as has been mentioned previously, 
contracts the red, orange, and yellow spaces unduly, 
and hence increases their illumination disproportion- 
ately. It is to be hoped that a corresponding set of 
measurements will soon be made on the normal spedtrum, 
furnished by a ruled plate. If we should multiply the 
luminosity of the colours in either kind of spedtrum by their 
extent or areas, we should obtain measures of the relative 
amounts of these several tints in white light. 
Fig. 3. 
Coloured Disc with Small Black-and-White Disc. 
Fig. 4. 
Coloured Disc with Small Black-and-White Disc in Rotation. 
By the simple method of rotating discs we can very 
roughly determine the second constant in the case of a 
coloured surface, for example, of paper tinted with Vermil- 
lion. A circular disc, about 6 inches in diameter, is cut 
from the paper, and placed on a rotation apparatus, as indi- 
cated in Fig. 3. On the same axis is fastened a double disc 
of black-and-white paper, so arranged that the proportions 
of black and white can be varied at will. When the whole 
is set in rapid rotation, the colour of the vermillion paper 
will of course not be altered, but the black and white will 
