i.] WOLLASTON'S DISCOVERY. 11 



was not absolutely continuous, as Newton had stated it to be, 

 but that it was interrupted by a series of dark spaces. 



This fundamental observation must here be given in Wol- 

 laston's own words, extracted from his communication to the 

 Royal Society : 1 



" The colours into which a beam of white light is separated by 

 refraction appear to me to be neither seven, as they are usually 

 seen in the rainbow, nor reducible by any means (that I can find) 

 to three, as some persons have conceived ; but that, by employing 

 a very narrow pencil of light, four primary divisions of the pris- 

 matic spectrum may be seen with a degree of distinctness that, 

 I believe, has not been described nor observed before. 



"If a beam of daylight be admitted into a dark room by a 

 crevice ^ of an inch broad, and received by the eye at the distance 



FIG. 5. Copy of Wollaston's Diagram. 



of ten or twelve feet, through a prism of flint glass free from veins, 

 held near the eye, the beam is seen to be separated into the four 

 following colours only : Red, yellowish-green, blue, and violet, in 

 the proportions represented in Fig. 5. 



" The line A that bounds the red side of the spectrum is somewhat 

 confused, which seemed in part owing to want of power in the eye 

 to converge red light. The line B, between red and green, in a 

 certain position of the prism, is perfectly distinct; so also are 

 D and E, the two limits of violet ; but c, the limit of green and 

 blue, is not so clearly marked as the rest ; and there are also, on 

 each side of this limit, other distinct dark spaces, /and g, either 

 of which in an imperfect experiment might be mistaken for the 

 boundary of these colours." 



1 Plnl. Trans. 1802, part i. p. 378. 



