478 HISTORY OF SCIENCE. 



his materials. He became Secretary to the Royal Society in 1806, 

 and shortly before his death he received the gold medal of the Society 

 for his platinum process. He bequeathed his wealth to objects calcu- 

 lated to promote science. 



The paper which brings Wollaston's name into our present chapter 

 appeared in the " Philosophical Transactions " for 1802. Its title im- 

 ports that it describes a new method for measuring refractive powers 

 by determining the angle of total reflection, and the following passages 

 are therefore merely incidental to its main subject. 



"I cannot conclude these observations on dispersion without re- 

 marking that the colours into which a beam of white light is separable 

 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 



FIG. 219. 



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

 narrow pencil of light, four primary divisions of the prismatic spectrum 

 may be seen with a degree of distinctness that, I believe, has not been 

 described or observed before. 



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

 .-Jfrth of an inch broad, and received by the eye at the distance 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. 219. 



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

 confused, which seems 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 lines, /and g, either of which in an imperfect 

 experiment might be mistaken for the boundary of these colours. 



" The position of the prism in which the colours are most clearly 

 divided is when the incident light makes about equal angles with its 

 two sides. I then found that the spaces A B, B c, c D, D E, occupied 

 by them, were nearly as the numbers 16, 23, 36, 25 



"By candlelight a different set of appearances may be distinguished. 





