464 The Constants of Colour , [October, 
different results on different days, though those of a single 
day may agree pretty well among themselves. In the 
appendix a peculiar photometer will be described, which 
has been contrived by the author for the purpose of 
comparing more accurately together the relative luminosity 
of different coloured surfaces, or that of coloured and white 
surfaces. 
But to resume our search for colour-constants. We may 
meet with two portions of coloured light, having the same 
degree of purity and the same apparent brightness, which 
nevertheless appear to the eye totally different ; one may 
excite the sensation of blue, the other that of red ; we say 
the tones are entirely different. The tone of the colour is 
then our third and last constant, or, as the physicist would 
say, the degree of refrangibility, or the wave-length of the 
light. It has previously been shown that the spedtrum 
offers all possible tones except the purples, well arranged 
in an orderly series ; and the purples themselves can be 
produced with some trouble, by causing the blue or violet 
of the spedtrum to mingle in certain proportions with the 
red. Rutherford’s automatic six-prism spedtroscope can 
very conveniently be employed for the determination of the 
tone. (See Fig. 5.) A peculiar eye-piece is to be used, 
which isolates a little slice of the spedtrum in its upper half, 
as indicated in Fig. 6. In the lower half of the field the 
fixed lines are seen, and the tone seledted as matching the 
colour under examination can be located by their aid. After- 
ward, if it is considered desirable, white light can be added 
to the spedtral tint, till it is subdued sufficiently to render 
exadt comparison possible. 
The experimental determination of the colour-constants 
is beset with a considerable amount of difficulty, even in the 
simplest cases, such as cardboards covered with pigments. 
The best mode of proceeding appears to be to call the lumi- 
nosity of white cardboard 100, and then to determine photo- 
metrically the comparative luminosity of the coloured 
cardboards. The measurement of the amount of white 
light refledted along with the coloured is still more trouble- 
some, and the result likely to be somewhat less exadt, while 
the determination of the tone, or third constant, is mode- 
rately easy under favourable circumstances. One of the uses 
of such determinations is, the production of a set of standard 
coloured discs with known constants, which can afterward 
be combined with each other, as well as with standard black 
or white discs, so as to generate at will, with ease and cer- 
tainty, an immense number of tints whose constants will be 
