PHYSICAL aNp LITERARY. 49 
dually greater in the prifimatic order ; ac- 
cording to obfervation, 
2. As the proportion between thefe 
intervals in red and violet can be afligned 
by experiment, and the proportion of their 
velocities inany medium likewife, by N° gos 
the velocity of the etherial pulfes in any 
medium, and their diftance from one an- 
other, may be thence computed by the 
following rule: “* Multiply the product 
* underthe velocities of the red and violet 
“rays by the difference of the intervals 
‘ of their fits ; then divide by the diffe-. 
“rence of the two produdts which are. 
* formed by multiplying the interval of 
© the fits in red by the velocity of the vio- 
** let, and the interval of the fits in violet 
“ by the velocity of red:” The quotient 
fhall exprefs the seit ef the ethereal 
pulfes *. 
Mota ln). ee ~ 43. THE 
+ * Let C denote the celerity of the etherea! pulfes, V the 
velocity of red light, and v that of violet, I and é the in- 
tervals of their fits, and D the perpendicular diftance of twa 
cceeding pulfes : It is plain, from the nature of the hypo- 
hefis, that I isto D as Vto C—V, and again, Dtoéas 
C—o 
