1873O Colours and their Relations. 89 



that it is polarised, and for indicating the character of its 

 polarisation. The colours are developed when certain 

 crystals and also certain organic substances are interposed 

 between the polariser and the analyser. In passing through 

 these interposed media, the light is more or less depolarised, 

 while the depolarising energy acts unequally on the different 

 waves, and is manifested unequally in different parts and 

 directions when the interposed medium is a crystal. The 

 result is the greater or less separation of the differently 

 coloured waves one from another, and that in such a manner 

 as, in many instances, to display the intimate internal 

 structure of the crystal, or other depolarising substance. 



The combined action of different colours when they fall 

 simultaneously on the retina is curious. An interesting 

 series of experiments, with a view to illustrate this action, 

 has been made by Prof. J. Clerk Maxwell, who has com- 

 municated the results to the Royal Society in a paper pub- 

 lished in the "Philosophical Transactions " for i860. By an 

 ingenious apparatus he contrived to bring three diverse pure 

 colours of the spectrum to bear on one point of the retina. 

 He ascertained that there is in the spectrum a central point, 

 which he describes as being about a fourth from E towards 

 F. This would make its wave-length on Angstrom's scale 

 5156*72. As Prof. Maxwell determined this point by means 

 of two flint glass prisms, allowance must be made for their 

 irrationality ; so that in all probability the exact position of 

 the central point is, in the normal spectrum, the mean green 

 ray, of which, as will be afterwards shown, the wave-length 

 is 5124*086 (reciprocal 1951*568). Prof. Maxwell has de- 

 tected a curious peculiarity of the rays at and near this 

 point, namely, that at the punctum ccecum, or yellow spot in 

 the retina, there is a greater insensibility to these rays than 

 to any others of the spectrum. 



By causing the rays from this central green point to fall 

 on the retina in conjunction with the rays from some point 

 in the red, Prof. Maxwell found that colours undistinguishable 

 from the intermediate pure orange and yellow of the spectrum 

 could be produced, the only difference being that these com- 

 pound tints are resolvable by the prism into their con- 

 stituent elements, while the pure tints of the spectrum are 

 not. In like manner it is always possible to select two 

 colours from somewhat distant points of the spectrum, 

 which will, when combined in certain proportions, produce 

 intermediate tints undistinguishable from one or other of 

 the remaining pure tints of the spectrum. 



The most remarkable effects, however, are those produced 



vol. in. (n.s.) n 



