and the Cause of Golour. 369 
of the division of the ray of light, it would seem from reasoning 
impossible that it should be so, netwithstanding that the com- 
pound coloured ray may be repassed through a prism without de- 
composition. Thus suppose, fer example, a triangular prism to 
have its side an inch in breadth. Then let a solar ray of light, 
of the breadth of the side, be passed through the upper angle, 
and received at the distance at which the spectrum will be two 
inches broad, Now it is net the whole ray which is at once di- 
vided, but every part; and as the degree of refraction depends 
upon the angle of the prism, the red ray which is refracted at 
the upper part of the prism, must be so in the same degree as 
the similar ray at the lower part, and consequently must be pa- 
rallel with that similar ray. The red ray of the upper part, there- 
fore, must in the spectrum be one inch from the red ray of the 
lower part, as well as when they first issue from the prism; and 
the remaining part.of the divided solar ray, which passed through 
the upper part of the prism, will be diffused through the whole 
upper inch of the spectrum. In the same manner the violet- 
coloured rays of the solar beam of the upper and lower parts of 
the prism must be parallel ; and, consequently, must be one 
iuch apart, and that of the lower part must fall exactly upon the 
red ray of the upper part ; and the divided ray of the lower part 
of the prism must be diffused through all the lower inch of the 
spectrum. Those parts of the solar ray which are between the 
extremes of the prism, will in:like manner be divided, and have 
their coloured divisions parallel to the similar coloured divisions 
of the extreme parts; and, consequently, the red ray of the dif- 
ferent parts will be diffused through the inch between the upper 
and lower red ray; and.the violet through the inch between the 
upper and lower extremes of the violet colour; and the inter- 
mediate colorific rays of every part,:hetween the extreme colours 
of that part. dn the centre of the whole spectrum, the ray will 
he compounded of every.colorific ray of the spectrum, and there 
will be no part of the spectrum, except its utmost limits, homo- 
geneous. In this case, for the sake of simplicity, the prism ‘has 
been supposed gne jinch, and the spectrum, two inches broad ; 
but, whatever mightbe the breadth, if the solar ray passed 
through the prism were of any considerable extent, the effect 
must ke, in its proportion, similar ; so that no part of the spec- 
trum except the perfect extremes could be homogeneous. Yet 
_ in all these cases, the ray of light seems simply divided into the 
common .prismatic colours ; ad each .of the .colours.seems as 
pure, and. distinct, as when the solar beam is more narrow ; and 
they are-all equally capable of being passed through: another 
prism.without decomposition. 
~ In.coufirmation of this reasoning, the effect of different phos- 
Vol.50, No,235, Nov, 1817. Aa phorescent 
