On ike Decompoution of Light. 1 C7 



now give a recapitulation of all that precedes, which T shall 

 endeavour to do with precision. 



White light is decomposed by refraction into an infinite 

 number of parts or rays. Thev have a different colour at 

 every point in the length of the spectrum ; and this colour 

 cannot be varied by a new refraction, if the sin)pIification of 

 the spectrum be at the degree to v\ hich Newton carried it. 

 Though in this simplified spectrum the lines of demarcation 

 between the colours are by no means verv perceptible, it is 

 impossible, however, to ascribe the gratiations of their tints 

 to one and the same law. V^arious observations establish the 

 existence of several distinct species of colours; and their di- 

 vision into seven classes, as given by Newton, agrees with 

 a great number of phasnomena. Yet some bodies, by their 

 peculiar refractive power, derange the spaces of the colours 

 in the spectrum ; for example, the green rays are, in some 

 instances, brought nearer the red, in others nearer the vio- 

 let ; which proves that the dispersion of the rays docs not 

 depend absolutely on their own nature. Such are the prin- 

 cipal observations it appeared to me necessary to make, in 

 order to show the present state of our knowledge ; and I 

 shall now proceed to examine the action of coloured bodies 

 upon ligbt. 



I have s^own* that all kinds of transparent bodies, of 

 different colours, which 1 have observed, transmit ultimately 

 only the red, or green, or violet rays. The progressive ab- 

 sorption 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 remarked, that, under 

 certain circumstances, the colours exhibited by refraction 

 were almost wholly these three, red, green, and violet; that 

 sometimes yellow appeared to arise from a mixture of red 

 and green, and blue from a mixture of green andvi<.let; 

 which my dial t» as well as the placing of certain coloured 



glasses 



• In my Memoir read at the Institute on the 1 1th of August 180^. 

 t Tliij dial it simply a circle cihibiting the tcvco primitive colours con- 

 L 4 formably 



