AS PERCEIVED BY THE EYE. 291 



the incident light, which entered through holes in the sides of the tubes, and was 

 reflected in the direction of the axis. In this way two beams oppositely polarized 

 were mixed, either of which could be coloured in any way by coloured glasses 

 placed over the holes in the tube. By means of a Nicol's prism placed at the 

 end of the tube, the relative intensities of the two colours as they entered the eye 

 could be altered at pleasure. 



(5.) Union of two coloured beams by means of a doubly-refracting Prism. 

 I am not aware that this method has been tried, although the opposite polari- 

 zation of the emergent rays is favourable to the variation of the experiment. 



(6.) Successive presentation of the different Colours to the Retina. 



It has long been known, that light does not produce its full effect on the eye 

 at once, and that the effect, when produced, remains visible for some after the light 

 has ceased to act. In the case of the rotating disc, the various colours become 

 indistinguishable, and the disc appears of a uniform tint, which is in some sense 

 the resultant of the colours so blended. This method of combining colours has 

 been used since the time of Newton, to exhibit the results of theory. The expe- 

 riments of Professor J. D. Forbes, which I witnessed in 1849, first encouraged me 

 to think that the laws of this kind of mixture might be discovered by special ex- 

 periments. After repeating the well-known experiment in which a series of colours 

 representing those of the spectrum are combined to form gray, Professor Forbes 

 endeavoured to form a neutral tint, by the combination of three colours only. For 

 this purpose, he combined the three so-called primary colours, red, blue, and 

 yellow, but the resulting tint could not be rendered neutral by any combination 

 of these colours ; and the reason was found to be, that blue and yellow do not 

 make green, but a pinkish tint, when neither prevails in the combination. It was 

 plain, that no addition of red to this, could produce a neutral tint. 



This result of mixing blue and yellow was, I believe, not previously known. It 

 directly contradicted the received theory of colours, and seemed to be at variance 

 with the fact, that the same blue and yellow paint, when ground together, do 

 make green. Several experiments were proposed by Professor Forbes, in order 

 to eliminate the effect of motion, but he was not then able to undertake them. 

 One of these consisted in viewing alternate stripes of blue and yellow, with a 

 telescope out of focus. I have tried this, and find the resultant tint pink as be- 

 fore.* I also found that the beams of light coloured by transmission through 

 blue and yellow glasses appeared pink, when mixed on a screen, while a beam of 

 light, after passing through both glasses, appeared green. By the help of the 

 theory of absorption, given by Herschel,| I made out the complete explanation 



* See however Encyc. Metropolitana, Art. " Light," section 502. f lb. sect. 516. 



VOL. XXI. PART II. 4 I 



