42 ON GOETHE'S SCIENTIFIC KESEAKCHES. 



and violet light, and light of ail intermediate colours. 

 Different kinds of light, or differently coloured lights, 

 produce, when mixed, derived colours, which to a cer- 

 tain extent resemble the original colours from which 

 they are derived ; to a certain extent form new 

 tints. White is a mixture of all the before-named 

 colours in certain definite proportions. But the pri- 

 mitive colours can always be reproduced by analysis 

 from derived colours, or from white, while themselves 

 incapable of analysis or change. The cause of the colours 

 of transparent and opaque bodies is, that when white light 

 falls upon them they destroy some of its constituents and 

 send to the eye other constituents, but no longer mixed 

 in the right proportions to produce white light. Thus a 

 piece of red glass looks red, because it transmits only red 

 rays. Consequently all colour is derived solely from a 

 change in the proportions in which light is mixed, and is, 

 therefore, a property of light, not of the coloured bodies, 

 which only furnish an occasion for its manifestation. 



A prism refracts transmitted light ; that is to say, de- 

 flects it so that it makes a certain angle with its original 

 direction ; the rays of simple light of different colours 

 have, according to Newton, different refrangibilities, and 

 therefore, after refraction in the prism, pursue different 

 courses and separate from each other. Accordingly a 

 luminous point of infinitely small dimensions appears, 

 when seen through the prism, to be first displaced, and 

 secondly, extended into a coloured line, the so-called 

 prismatic spectrum, which shows what are called the pri- 

 mary colours in the order above-named. If, however, you 

 look at a broader luminous surface, the spectra of the 

 points near the middle are superposed, as may be seen 

 from a simple geometrical investigation, in such pro- 

 portions as to give white light, except at the edges, where 

 certain of the colours are free. This white surface appears 

 displaced, as the luminous point did ; but instead of being 



