374 Prof. Maxwell on TJieory of Three Primary Colours. [May 17, 



the spectrum was exhibited by three coloured images, which, when 

 superposed on the screen, gave an artificial representation of the spec- 

 trum. 



Three photographs of a coloured ribbon taken through the three 

 coloured solutions respectively, were introduced into the camera, giving 

 images representing the red, the green, and the blue parts separately, 

 as they would be seen by each of Young's three sets of nerves 

 separately. When these were superposed, a coloured image was 

 seen, which, if the red and green images had been as fully photo- 

 graphed as the blue, would have been a truly- coloured image of 

 the ribbon. By finding photographic materials more sensitive to the 

 less refrangible rays, the representation of the colours of objects might 

 be greatly improved. 



The speaker then proceeded to exhibit mixtures of the colours of 

 the pure spectrum. Light from the electric lamp was passed through 

 a narrow slit, a lens and a prism, so as to throw a pure spectrum on a 

 screen containing three moveable slits, through which three distinct 

 portions of the spectrum were suffered to pass. These portions were 

 concentrated by a lens on a screen at a distance, forming a large, 

 uniformly coloured image of the prism. 



When the whole spectrum was allowed to pass, this image was 

 white, as in Newton's experiment of combining the rays of the spec- 

 trum. When. portions of the spectrum were allowed to pass through 

 the moveable slits, the image was uniformly illuminated with a mix- 

 ture of the corresponding colours. In order to see these colours 

 separately, another lens was placed between the moveable slits and the 

 screen. A magnified image of the slits was thus thrown on the screen, 

 each slit showing, by its colour and its breadth, the quality and quan- 

 tity of the colour which it suffered to pass. Several colours were thus 

 exhibited, first separately, and then in combination. Red and blue, 

 for instance, produced purple ; red and green produced yellow ; blue 

 and yellow produced a pale pink ; red, blue, and green produced 

 white ; and red and a bluish green near the line F produced a colour 

 which appears very different to different eyes. 



The speaker concluded by stating the peculiarities of colour-blind 

 vision, and by showing that the investigation into the theory of colour 

 is truly a physiological inquiry, and that it requires the observations 

 and testimony of persons of every kind in order to discover and explain 

 the various peculiarities of vision. 



[J. C. M.] 



