76 



siderable, and the latter a very large proportion of green light, to the 

 predominance of which in the joint reflected beam its tint is owing. 

 So also, when blue and yellow liquids (not acting chemically on each 

 other) are mixed, as in water-colour drawings, greens, sometimes 

 very lively ones, are produced. In these cases the yellow absorbs 

 almost all the whole of the incident blue, indigo, and violet light, and 

 the blue a very large proportion of the red, orange, and yellow, both 

 allowing much green to pass ; and to this, rather than to a mixture of 

 the other rays, the resulting tint is due. 



In the light transmitted by cuprate of ammonia of a certain thick- 

 ness, the red, orange, yellow, and green are wholly extinguished, 

 while the blue, indigo, and violet are allowed to pass . The result is 

 the fullest and bluest blue it is possible to obtain. From this result, 

 compared with that derived from the analysis of natural yellows, it 

 follows that the union on the retina of the yellowest yellow, and the 

 bluest blue, in such proportions that neither shall be in excess, so as 

 to' tinge the resulting light either yellower blue, is not green, but 

 white. The same conclusion follows from dividing the spectrum 

 into two, the one portion containing all the less refrangible rays up 

 to the limit of the green and blue, the other all the remaining rays. 

 If the blue portion be suppressed, and the remainder reunited by a 

 refraction in the opposite direction, the resulting beam is yellow, if 

 the other, blue, both vivid colours but if neither, white of course, 

 and not green, results from the exact recomposition of the original 

 white beam. 



It may be objected to this, that in the complementary colours 

 exhibited by doubly-refracted pencils in polarized light, yellow is 

 often found to be complementary to purple, and blue to orange. 

 But in neither of these pairs of colours is the spectrum divided in 

 the manner above indicated ; and, moreover, in many instances yellow 

 and blue are found as complementary colours in the oppositely 

 polarized pencils ; of which examples will be found in the scale of 

 tints produced by sulphate of barytes in my paper " On the Action 

 of Crystallized Bodies in Homogeneous Light" (Phil. Trans. 1820, 

 Table I.). " Rich yellow" appears also as opposed to "full blue" 

 in the scale of complementary tints exhibited by mica in my " Trea- 

 tise on Light" (Encyc. Metrop., art. 507). It is not asserted that 

 either a good yellow or a good blue cannot be produced otherwise 



