EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 145 



(4) Union of two beams by means of a transparent surface, which reflects 



the first and transmits the second. 



The simplest experiment of this kind is described by M. Helmholtz. He 

 places two coloured wafers on a table, and then, taking a piece of transparent 

 glass, he places it between them, so that the reflected image of one apparently 

 coincides with the other as seen through the glass. The colours are thus mixed, 

 and, by varying the angle of reflection, the relative intensities of the reflected 

 and transmitted beams may be varied at pleasure. 



In an instrument constructed by myself for photometrical purposes two re- 

 flecting plates were used. They were placed in a square tube, so as to polarize 

 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 

 polarization 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 time 

 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 com- 

 bining colours has been used since the time of Newton, to exhibit the results 

 of theory. The experiments 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 experiments. After repeating the well-known experiment 

 in which a series of colours representing those of the spectrum are combined 



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