37 ^ Dr. Wollaston's Method of examining 
By comparison of this table with the order of refractive 
powers, as contained in the first table, it will be seen how little 
correspondence there is between them ; and, accordingly, how 
numerous are the combinations by means of which a pencil of 
rays that passes through two media, may be made to deviate 
without dispersion of its colours. 
I cannot conclude these observations on dispersion, without 
remarking that the colours into which a beam of white light is 
separable by refraction, appear to me to be neither 7, as they 
usually are seen in the rainbow, nor reducible by any means 
(that I can find) to 3, as some persons have conceived; but 
that, by employing a very narrow pencil of light, 4 primary 
divisions of the prismatic spectrum may be seen, with a degree 
of distinctness that, I believe, has not been described nor ob- 
served before. 
\ 
If a beam of day-light be admitted into a dark room by a crevice 
of an inch broad, and received by the eye at the distance of 
10 or 12 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. 3. 
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 experi- 
ment, might be mistaken for the boundary of these colours. 
