Dr BREWSTER on a New Analysis of Solar Light. 125 



dependent existence in the spectrum ; and that the prism is inca- 

 pable of decomposing that part of the spectrum (of four colours) 

 which it occupies. This unequivocal result of a simple expe- 

 riment at once saps the foundation of the prismatic analysis of 

 light. Sir ISAAC NEWTON, resting on the indications of the 

 prism, concluded that green and orange were simple colours, and 

 in general, " that to the same degree of refrangibility ever be- 

 longed the same colour, and to the same colour ever belonged 

 the same degree of refrangibility ;" but it is now obvious that 

 certain blue and yellow rays, and certain red and yelloiv rays, have 

 the very same refrangibility, so that, in the same medium, refran- 

 gibility is not a test of colour, nor colour a test of refrangibility. 

 These views were confirmed by experiments made by Mr 

 HERSCHEL, and printed in the same volume of the Transactions ; 

 but, in referring to them five years afterwards, in his Treatise on 

 Light, he regarded them as liable to formidable objections. 

 " This idea," says he (the inability of the prism to analyze light), 

 has been advocated by Dr BREWSTER, in a paper published in 

 the Edinburgh Philosophical Transactions, Vol. XI., and the same 

 conclusion appears to follow from other experiments published 

 in the same volume of that collection. According to this doc- 

 trine, the spectrum would consist of at least three distinct spec- 

 tra of different colours, red, yellow, and blue, overlapping each 

 other, and each having a maximum of intensity at those points 

 where the compound spectrum has the strongest and brightest 

 tint of that colour. It must be confessed, however, that this doc- 

 trine is not without its objections ; one of the most formidable 

 of which may be drawn from the curious affection of vision occa- 

 sionally (and not very rarely) met with in certain individuals who 

 distinguish only two colours which, (when carefully questioned 

 and examined, by presenting to them, not the ordinary compound 

 colours of painters, but optical tints of known composition), are 

 generally found to be yellow and blue" 



VOL. XII. PART I. R 



