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flexibility, were, as we have seen, discovered by 
Newton ; and the same incomparable philosopher 
pointed out also the difference in their illuminating 
power. The most luminous of the prismatic rays, 
says he, are the yellow and orange ; next to these, 
the red and green ; the blue is fainter ; and the in- 
digo and violet are still more dark and faint. The 
most luminous and fulgent part is in the brightest 
yellow, where it inclines more to the orange than to 
the green *. Dr Herschell, by causing the prismatic 
rays to fall successively on an object placed beneath 
a lens, found the illuminating power to be small in the 
red ray, greater in the orange, and greatest in the 
centre of the spectrum, between the yellow and green. 
From the full deep green, the illuminating power de- 
creased very sensibly, being in the blue nearly on a 
par with that of the red, and in the violet it was 
least of all. By other experiments, he ascertained 
that this property resides in peculiar rays, which are 
distinct from those that impart heat ; for the illumi- 
nating rays are capable of being transmitted through 
substances by which the heating rays are stopped, and 
vice versa ; facts which the experiments of Scheele 
had before shewn to be the case with the heat and 
light which radiate from a common fire f. To these 
facts we may add, that the light of the moon, though 
composed of all the prismatic rays, does not, even 
when concentrated by a lens, impart the least sensi- 
ble heat. Hence then the rays, which render objects 
* Optics, B. i. prop. /. 
t On Air and Fire, p. 70. 
