ON THE THEORY OF COMPOUND COLOURS. 415 



colours from red to green-yellow (X = 2082) were complementary to colours ranging 

 from green-blue (X=1818) to violet, and that the colours between green-yellow 

 and green-blue have no homogeneous complementaries, but must be neutralized 

 by mixtures of red and violet. 



M. Helmholtz also gives a provisional diagram of the curve formed by the 

 spectrum on Newton's diagram, for which his experiments did not furnish him 

 with the complete data. 



Accounts of experiments by myself on the mixture of artificial colours by 

 rapid rotation, may be found in the Transactions of the Royal Society of 

 Edinburgh, Vol. xxi. Pt. 2 (1855); in an appendix to Professor George Wilson's 

 work on Colour-Blindness ; in the Report of the British Association for 1856, 

 p. 12; and in the Philosophical Magazine, July 1857, p. 40' These experiments 

 shew that, for the normal eye, there are three, and only three, elements of 

 colour, and that in the colour-blind one of these is absent. They also prove 

 that chromatic observations may be made, both by normal and abnormal eyes, 

 with such accuracy, as to warrant the employment of the results in the calcu- 

 lation of colour-equations, and in laying down colour-diagrams by Newton's rule. 



The first instrument which I made (in 1852) to examine the mixtures of 

 the colours of the spectrum was similar to that which I now use, but smaller, 

 and it had no constant light for a term of comparison. The second was 6^ feet 

 long, made in 1855, and shewed two combinations of colour side by side. I have 

 now succeeded in making the mixture much more perfect, and the comparisons 

 more exact, by using white reflected light, instead of the second compound 

 colour. An apparatus in which the light passes through the prisms, and is 

 reflected back again in nearly the same path by a concave mirror, was shewn 

 by me to the British Association in 1856. It has the advantage of being 

 portable, and need not be more than half the length of the other, in order 

 to produce a spectrum of eqTial length. I am so well satisfied with the working 

 of this form of the instrument, that I intend to make use of it in obtaining 

 equations from a greater variety of observers than I could meet with when I 

 was obliged to use the more bulky instrument. It is difficult at first to get 

 the observer to believe that the compound light can ever be so adjusted as to 

 appear to his eyes identical with the white light in contact with it. He has to 

 learn what adjustments are necessary to produce the requisite alteration under 

 all circumstances, and he must never be satisfied till the two parts of the 

 field are identical in colour and illumination. To do this thoroughly, implies 



