264 A. M. Mayer— History of Young’s 
“Tf we mix together, in proper proportions, any substances 
exhibiting these colours in their greatest purity, and place the 
mixture in a light sufficiently strong, we obtain the appearance 
of perfect whiteness; but in a fainter light the mixture is grey, 
or of that hue which arises from a combination of white and 
to revolve with such rapidity, that the whole may assume the 
appearance of a single tint, or a combination of tints resulting 
rom the mixture of the colour : 
not the blue of the Segre for four parts of green and one o 
violet make a blue differing very little from green; while the 
