198 Dr Brewster on a Nets Analysis of Solar Light. 



that part of the spectrum (of four colours) which it occupies. 

 This unequivocal result of a simple experiment 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 belonged 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 yellow rays, have the very 

 same refrangibility, so that, in the same medium, refrangibility 

 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 Transac- 

 tions; 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 collec- 

 tion. According to this doctrine, the spectrum would consist 

 of at least three distinct spectra 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 doctrine is not without its ob- 

 jections ; one of the most formidable of which may be drawn 

 from the curious affection of vision occasionally (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.' 1 '' 



To these remarks Mr Herschel has added an illustrative 

 figure of the spectrum, in which it is made to consist of four 

 colours, red, yellow, blue, and violet ; the red extending to the 

 middle of the yellow, the yellow beginning at the extremity of 

 the orange and terminating at the indigo, the blue beginning 

 at the middle of the yellow, and terminating at the end of the 



