385. Flint-glass, which contains in its composition
oxide of lead, has a greater dispersive power than crown-

Srlass, which does not contain this ingredient. If, there-
ore, ABC (378) be a prism of flint-glass, and A F C a
similar one of crown-glass, though the spectrum will dis-
appear, and the luminous spot, W, be reproduced, it will

«iTer tbe whole Fpectrum ? Wb<?re le the red most IntenBc 1 Where the yellow 7 How
Is thii ILtufttral4id in th^ iicure I Wlifti m said of the composition of each of the colors
«f the spectJTum ten Ides tltc thre* primfLry rnys 1 What is the orange chiefly composed
of? 384, Why is lh« green anmeJira^s oeUled the mean ra^ 7 Does it always occupy
th« mid^jlti of the flpectrum 1 Will the lengtti of the spectrum depend upon tb« natura
tif the prism B&c?d f What 3b eaid of the Hiectrum produced by a prism of oil of cassia?
What IB m can I by th e di^fterifivi fouptr o f a prism 1 385. What is said of the dispersiya
pDwtr of a prtam vninjie of Hint-gUsa, m compared with one of crown-glaRsI If two
pristDfl, tiiQllar In torm^hw. one of ilini-gUss and the other of crown-glass, are comT



OPTICS.
945



not be composed of pure white light, as was the case

when two prisms of the same kind of glass were used
(379,) but colored on one side with red, and on the other
with purple. This is occasioned by the unequal disper-
sive power of the two kinds of glass producing spectra
of unequal lengths, though the mean ray is equally re^
fracted by both, and therefore the luminous spot produced
just where it was before.
