1876.J The Constants of Colour. 465 
known. If we make a record of the constants involved in 
such experiments we can afterward reproduce the tints just 
as they originally were, or alter them to any desirable 
extent. To carry out the letter of this it will of course be 
necessary to view the standard discs under similar illumina- 
tions at different times— -a point which can be secured with 
the aid of the photometer above referred to. The standard 
discs can also be used for building up a set of standard 
charts, containing a vast variety of tints of known compo- 
sition, arranged methodically with regard to purity, lumi- 
nosity, and tone. These matters will be considered at some 
length in a separate chapter, and are now only hinted at as 
a justification for the trouble we have been at in defining 
the constants of colour. 
Fig. 6. 
Eyepiece for Isolating the Tints of the Spedtrum. 
There is another point to be touched on in this connection. 
One of the most noticeable things about colours is their dif- 
ference in intensity. Colours are intense when they excel 
both in purity and brightness ; for it is quite evident that, 
however pure the coloured light may be, it still will produce 
very little effeCt on the eye if its total quantity be small ; 
and, on the other hand, it is plain that its aCtion on the 
same organ will not be considerable if it is diluted with 
much white light. Purity and brightness, or luminosity, 
are, then, the factors on which intensity depends. We 
shall see hereafter that this is strictly true only within cer- 
tain limits, and that an inordinate increase of luminosity is 
attended with a loss of intensity of hue. 
Having defined the three constants of colour, it will be 
interesting to inquire into the sensitiveness of the eye in 
these directions. This subject has lately been studied with 
VOL. vi. (n.s.) 2 R 
