ON THE THEORY OF THREE PRIMARY COLOURS. 449 



in every case. He had also examined all the colours of the spectrum with 

 the same result. 



The experiments with pigments do not indicate what colours are to be 

 considered as primary ; but experiments on the prismatic spectrum shew that 

 all the colours of the spectrum, and therefore all the colours in nature, are 

 equivalent to mixtures of three colours of the spectrum itself, namely, red, 

 green (near the line E], and blue (near the line (7). Yellow was found to be 

 a mixture of red and green. 



The speaker, assuming red, green, and blue as primary colours, then exhi- 

 bited them on a screen by means of three magic lanterns, before which were 

 placed glass troughs containing respectively sulphocyanide of iron, chloride of 

 copper, and ammoniated copper. 



A triangle was thus illuminated, so that the pure colours appeared at its 

 angles, while the rest of the triangle contained the various mixtures of the 

 colours as in Young's triangle of colour. 



The graduated intensity of the primary colours in different parts of the 

 spectrum was exhibited by three coloured images, which, when superposed on 

 the screen, gave an artificial representation of the spectrum. 



Three photographs of a coloured ribbon taken through the three coloured 

 solutions respectively, were introduced into the camera, giving images represent- 

 ing 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 super- 

 posed, a coloured image was seen, which, if the red and green images had 

 been as fully photographed 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 spectrum. When portions 

 of the spectrum were allowed to pass through the moveable slits, the image was 



VOL. I. 57 



