C 264 ] 
equal ditlances ; and therefore the intervals of die fits 
of reflexion and tranfmiftion, if they arife in this 
manner, as Sir Ifaac conjectures, would be all equal : 
but if the red move fwirteft, the violet floweft, and 
the intermediate colours with intermediate velocities, 
it is plain, that the fame pulfes muft overtake the 
violet fooneft, the other colours in their order, and 
lalt of all the red ; that is, the intervals of the fits 
muff be leaft in the violet, and gradually greater in 
the prifmatic order, agreeably to obfervation. 
6. Let c denote the velocity of the aethereal pulfes, 
V the velocity of red light, and U that of violet ; 1 
and y the intervals of their fits, and D die diftance 
between two fucceeding pulfes : it is plain, from the 
nature of Newton’s hypothefis, that T is to D , as V 
to C — V : and again, D to J as C — U to U: there- 
fore, ex a quo, I is to J, as CV — VU to CU — VU y 
from which we have the equation C = 
Therefore, as the proportion between the intervals of 
the fits in red and violet, can be affigned by experi- 
ment, and the proportion of their velocities in any 
medium likewiL, by Art. 4. the velocity of the aathe- 
real pulfes may be eafily computed. The velocities 
of the red and violet in air are nearly as 78 and 
77. In the celeftial fpaces they are lefs, but almoft 
in the fame proportion j the intervals of their fits are 
by experiment as 100 and 63 •j-', from whence, by 
the canon now laid down, the velocity of the aethereal 
pulfes in the celeftial lpace is found to be to that of 
red 
j; Newton’s Optics, Lib, II. lbrt I. Obf. 14. 
