PHYSICAL anp LITERARY. 51 
of the interval of the fits in red, and there- 
fore does not much exceed ,,;35,; of an 
inch *, 
45» THE velocity of the ethereal pulfes 
being determined, as above, from the in- 
tervals of the fits in the two extreme co- 
lours, as found by experiment, the inter- 
vals of the fits in the fix intermediate rays 
may be calculated from theory; for the 
interval in any one colour muft be, to 
that in red, as a product under the velo- 
city of the given colour and the excefs of 
the velocity of the ethereal pulfes above 
that of red, isto a product under the ve- 
locity of red and the excefs of the velocity 
of the ethereal pulfes above that of the gi- 
ven colour: But, even upon the fuppofi- 
Vtion of the trath of our theory, an exact 
coincidence between calculation and ex- 
periment is not to be expected till the ve- 
Jocities of the fays be more accurately de- 
_ termined. 
_ 46. Upon the hypothefis of the diffe- 
4 
it * See the table of the thicknefs of colcured plates in 
Newt, opt. part 2. book 2, ; 
" .. rent 
