74 
PEOFESSOE J. CLEEK JMAXWELL OX 
§ XI. The Spectrum as laid down on NEWTO^'’s Diagram. 
The curve on which these points lie has this striking feature, that two portions of it 
are nearly, if not quite, straight lines. One of these portions extends from (24) to (46), 
and the other from (48) to (64). The colour (20), and those beyond (64), are not far 
from the line joining (24) and (68). The spectrum, therefore, as exhibited in Xewtox’s 
diagram, forms two sides of a triangle, with doubtful fragments of the third side. Xow 
if three colours in Newton’s diagram lie in a* straight line, the middle one is a compound 
of the two others. Hence all the colours of the spectrum may be compounded of those 
which lie at the angles of this triangle. These correspond to the following colours : — 
Table VII. 
Scale. 
Wave-length. 
Index 
in water. 
Wave-length 
in water. 
E 
Scarlet 
. 24 
2328 
1-332 
1-747 
G 
Green . 
. 46f 
1914 
1-334 
1-435 
B 
Blue . 
. 64i 
1717 
1-339 
1-282 
All the other colours of the spectrum may be produced by combinations of these ; and 
since all natural colours are compounded of the colours of the spectrum, they may be 
compounded of these three primary colours. I have strong reason to believe that these 
are the three primary colours corresponding to three modes of sensation in the organ of 
vision, on which the whole system of colour, as seen by the normal eye, depends. 
§ XII. Besults fownd hy a second Observer. 
We may now consider the results of three series of observations made by myself (J.) 
as observer, in order to determine the relation of one observer to another in the percep- 
tion of colour. The standard colours are connected by the following equation, as deter- 
mined by six observations : — 
18-l(24)-l-27-5(44)+37(68)=:W* (17.) 
The average errors in these observations were — 
Table VIII. 
R_j_G+B, -95 
showing that in this case, also, the power of distinguishing colour is more to be depended 
on than that of distinguishing degrees of illumination. 
The average error in the other observations from the means was ‘64 for red, -76 for 
green, and 1’02 for blue. 
E, 
•28 
g+b. 
•83 
G- 
-B, 
•83 
G, 
-83 
B -fE, 
•42 
B- 
-B, 
•28 
B, 
-16 
E-f G, 
•95 
E- 
-G, 
•72 
