PHYSICAL, AND LITERARY. 45 



f Multiply the produ(fl under the velocities 

 " of the red and violet rays by the difference 

 « of the intervals of their fits j then divide 

 " by the difference of the two produds 

 « which are formed by multiplying the in- 

 f terval of the fits in red by the velocity 

 »< of the violet, and the interval of the fits 

 ** in violet by the velocity of red :" the quo- 

 tient (hall exprefs the velocity of the ethereal 

 pulfes *. 



43. The velocities of the red and violet 

 in air, are, by the above eftimation, as 78. 

 and yy -f- ; and the intervals of their fits are, 

 by experiment i, as 100 and 63 : from 

 whence, by the canon now laid down, the 

 velocity of the ethereal pulfes is found to 

 be. to that of red light, as 79763 to 78000. 



As 



• Let C denote the celerity of the ethereal pulfes, V the 

 velocity of red light, and i; that of violet, I and / the in- 

 tervals of their fits, and D the perpendicular diftance of two 

 Succeeding pulfes : it is plain, from the nature of the hypo- 

 thefis, that I is to D as V to C — V, and again, D to / as 

 C — V to 1; ; therefore, ex aquo, 1 is to i as CV — Vi; to 

 Co/ — V'u : from which arifes the equation fUTx Vw 



1 X <v—i X V. 



•f In the celeftial medium they are lefs, [No 40.] but very 

 yearly in the fame proportion. 



X Newt. Opt. book 2. p. I. obfervat. i^. 



