48 ESSAYS anp OBSERVATIONS 
thefe, as the fine of incidence to the fine of 
refraction, when a ray paffes from air in- 
to the given medium *, 
41. WHILE the differently-coloured 
rays are fuppofed to move with one com= 
mon velocity, any pulfes, excited in the 
ethereal medium, muft overtake them at e- 
qual diftances; and therefore the inter- 
vals of reflexion and tran{miflion, if they 
arife in this manner, as Sir Ifaac New- 
ton conjectures, would bé all equal: But, 
if the red move {wifteft, the violet floweft, 
and the intermediate colours with inter- 
mediate velocities, it is plain, that the 
fame pulfes muft overtake the violet foon- 
eft, the other colours in their order, and, 
laft of all, the red; that is, the intervals. 
of the fits muft be leaft in violet, and gra- 
dually 
of S, DGC to $, DCG; wherefore DR is to EV in the 
fubduplicate ratio of S, DCR X S, DGC to 8S, DCGX 
S, DRC; that is, ‘* The velocities before incidence are 
‘* nearly in the direct fubduplicate ratio of thefe fines 
‘* and the reciprocal fubduplicate ratio of the fines of 
«© the exceifes of the common angle of incidence above 
«« the feveral angles of refradtion.” 
* Newt. princip. lib. 2. prop: 10. 
