DECOMPOSITION OF LIGUT. 



4? 



lour at every point in the length of the spectrum, and Jhis 

 .■colour cannot be varied by a new refraction, if the simpii r 

 fixation of the spectrum be at the degree to which Newton 

 carried it. Though in the spectrum thus simplified the lines which are really 

 of demarcation between the colours are by no means very dlstinct » 

 perceptible; it is impossible to ascribe the gradations of 

 their tints to one and the same law. Numerous observa- 

 tions establish the existence of several distinct species of 

 colours; and their division jnto seven classes, as given by 

 .Newton, agrees with a great number of phenomena. Yet but not always 

 some substances, by their peculiar refractive power, derange a "| d m an 

 the spaces of the colours in the spectrum; so that, for ex- though always 

 ample, the green rays are in some instances brought nearer ' n same ot r 

 the red, in others nearer the violet. This proves, that the 

 dispersion of the rays does not depend absolutely on their 

 own nature. These are the principal observations it ap» 

 pearcd to me necessary to make, in order to shew the pre- 

 sent state of our knowledge; and I shall now proceed to 

 -examine the action of coloured bodies upon light. 



I have formerly shewn, that all kinds of transparent bo- fhe yellow and 

 .dies, of different colours, which I have observed, transmit blue ra y s disap- 

 ultimately only on the red, or green, or violet rays. The 

 progressive absorption never finishes by any other colours, 

 .and I long sought in vain for a substance, in which the final 

 absorption should be of the yellow or blue rays. 



Such a result could not fail to excite my attention. I re- g . , 



marked, that, uuder certain circumstances, the colours ex- green and violet 

 bibited bv refraction were almost wholly these three, red, t^ f hLbUed 



J ? ; by refraction. 



green,, and violet: that sometimes yellow appeared to arise T > n 

 from a mixture of red and green, and blue from a mixture compounded of 

 of green and violet; which my dial,* as well as the placing se ' 

 of certain coloured glasses on each other, indicated as pos- 

 sible. I perceived too, that the tints of the seven orders 

 of colours might be imitated by the three primitive colours 

 alone which I have mentioned. + This was sufficient to 



suggest 



* This dial is simply a circle exhibiting the seven primitive co- 

 lours conformably to the ideas of Newton. See Optics, Book L 

 jProb. 2. The author has explained the principles ot the construc- 

 tion of this dial, and its leading properties, in his work. 



f This proposition of three primitive colours is very different from 

 C 2 .that 



