AS PERCEIVED BY THE EYE. 



29 7 



in M. Helmholtz's former experiments, and the facility of forming combinations 

 is much increased. In this memoir the mathematical theory of Newton'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, i. e., 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 : — 



Colour. 



Wave-length. 



Complementary 

 Colour. 



Wave-length. 



Ratio of 

 Wave-lengths. 



Red, . . . 



2425 



Green-blue, . 



1818 



1-334 



Orange, . 



2244 



Blue, . . . 



1809 



1-240 



Gold-yellow, 



2162 



Blue, . . . 



1793 



1-206 



Gold-yellow, 



2120 



Blue, . . . 



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 wa 



ve-lengths are e 



xpressed 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. 



