AS PERCEIVED BY THE EYE. 297 
in M. HeLmMuoutz’s former experiments, and the facility of forming combinations 
is much increased. In this memoir the mathematical theory of NEwron’s circle, 
and of the curve formed by the spectrum, with its possible transformations, is com- 
pletely stated, and the form of this curve is in some degree indicated, as far as 
the determination of the colours which lie on opposite sides of white, and of those 
which lie opposite the part of the curve which is wanting. The colours between 
red and yellow-green are complementary to colours between blue-green and violet, 
and those between yellow-green and blue-green have no homogeneous comple- 
mentaries, but must be neutralized by various hues of purple, 7.¢., mixtures of 
red and violet. The names of the complementary colours, with their wave- 
lengths in air, as deduced from FrAUNHOFER’s measurements, are given in the fol- 
lowing table :— 
Ratio of 
Wave-lengths. 
Complementary 
Colour. Wave-length. Gales 
Wave-length. 
Red, ... 2495 Green-blue, . 1818 1-334 
Orange, . . 2244 Blne ness 1809 1-240 
Gold-yellow, 2162 Blue 1793 1-206 
Gold-yellow, 2120 Blne,) sy) 1781 1:190 
Yellow, . . 209'5 Indigo-blue, . 1716 1-221 
Yellow, . . 2085 Indigo-blue, . 1706 1-222 
Green-yellow, 2082 Violet, . . 1600— 1:301 
(The wave-lengths are expressed in millionths of a Paris inch.) 
(In order to reduce these wave-lengths to their actual length in the eye, each 
must be divided by the index of refraction for that kind of light in the medium in 
which the physical effect of the vibrations is supposed to take place.) 
' Although these experiments are not in themselves sufficient to give the com- 
_ plete theory of the curve of homogeneous colours, they determine the most im- 
_ portant element of that theory in a way which seems very accurate, and I cannot 
_ doubt that when a philosopher who has so fully pointed out the importance of 
general theories in physics turns his attention to the theory of sensation, he will 
at least establish the principle that the laws of sensation can be successfully in- 
vestigated only after the corresponding physical laws have been ascertained, and 
that the connection of these two kinds of laws can be apprehended only when 
_ the distinction between them is fully recognised. 
