ON THOMAS TOUN&'S THEOET OF COLOUR-VISION. 



433 



Fig. 2. 



we shall evidently have two spectra at the slit Sj superposed one over the 

 other. An eye placed close 



before the slit Si sees a Fig. 1. 



picture like fig. 2. A little 

 consideration will make it 

 plain to us that the colour- 

 ed parts in the figure are 

 the two faces of the prism 

 shining with that light, 

 which coming from them 

 passes through the slit S i 

 to our eye. 



Let us consider the 

 ejffect of placing the ice- 

 land-spar between the slit 

 and the object-glass of the 

 collimator. This effect will 

 be that in general we shall 

 have two pairs of spectra 

 at S,, one pair due to sUt S2, the other due to S3, and that the lights of 

 one pair belonging to the same slit will be polarised perpendicularly to 

 each other. It is evident that an eye placed close 

 before the slit Sj will see the same picture as 

 before, but now we do not see in each half 

 monochromatic light, but a light which is the 

 resultant of two component lights, and now the 

 object of interposing the Nicol between the slit 

 and the source of light is simply to vary the ratio 

 of these two components to each other. When 

 the iceland-spar is quite close to the slit we have 

 monochromatic light in the corresponding half 

 of the picture. 



In this manner we can compare mono- 

 chromatic light with monochromatic, a mixture 

 of two components with monochromatic and 

 two such mixtures. This comparison consists in 

 producing the same shade of colour and the same 

 intensity in each half of our picture, which gives us a colour-equation. A 

 great number of such colour-equations was made by those persons whom 

 we examined. 



The coefficients and variables of the equation are given by the position 

 of the collimators, the distances between the iceland-spars and the slits, 

 the positions of the Nicols, and the micrometricaUy measured width of the 

 two bilateral slits. The source of light was a specially constructed gas- 

 lamp, so that our direct results had, of course, reference to the prismatic 

 spectrum of a certain kind of gaslight. But in order that they should 

 have a general character we calculated what they would have been if we 

 had employed a diffraction spectrum of sunhght. Of course I cannot 

 think of entering upon the description of a great many and very interest- 

 ing details of our proceeding. I shall therefore pass over directly to the 

 results of our investigation. 



§ 3. (a) There are persons who can distinguish no different shades of 

 colour, and therefore the world, as far as colour is concerned, appears to 



1886. F r 



(The two parts of this figure, 

 which are differently hatch- 

 ed, are supposed to be differ- 

 ently coloured.) 



