436 ON THE THEORY OF COMPOUND COLOURS. 



which colours chromatically identical with the other colours of the spectrum 

 may be produced. The exact position of the red and blue is not yet ascer- 

 tained ; that of the green is from E towards F. 



The orange and yellow of the spectrum are chromatically equivalent to 

 mixtures of red and green. They are neither richer nor paler than the corre- 

 sponding mixtures, and the only difference is that the mixture may be resolved 

 by a prism, whereas the colour in the spectrum cannot be so resolved. This 

 result seems to put an end to the pretension of yellow to be considered a 

 primary element of colour. 



In the same way the colours from the primary green to blue are chro- 

 matically identical with mixtures of these ; and the extreme ends of the spectrum 

 are probably equivalent to mixtures of red and blue, but they are so feeble 

 in illumination that experiments on the same plan with the rest can give no 

 result, but they must be examined by some special method. When observations 

 have been obtained from a greater number of individuals, including those whose 

 vision is dichromatic, the chart of the spectrum may be laid down independently 

 of accidental differences, and a more complete discussion of the laws of the 

 sensation of colour attempted. 



POSTSCRIPT. 

 [Received May 8, Read May 24, I860.] 



Since sending the above paper to the Royal Society, I have obtained 

 some observations of the colour of the spectrum by persons whose vision is 

 " dichromic," and who are therefore said to be " colour-blind." 



The instrument used in making these observations was similar in principle 

 to that formerly described, except that, in order to render it portable, the rays 

 are reflected back through the prisms, nearly in their original direction ; thus 

 rendering one of the limbs of the instrument unnecessary, and allowing the 

 other to be shortened considerably on account of the greater angular dispersion. 

 The principle of reflecting light, so as to pass twice through the same prism, 

 was employed by me in an instrument for combining colours made in 1856, 

 and a reflecting instrument for observing the spectrum has been constructed 

 independently by M. Porro. 



