44 ESSAYS AND OBSERVATIONS 



thefe, as the fine of incidence to the fine of 

 refradion, when a ray paffes from air into 

 the given medium ^. 



41. While the differently-coloured rays 

 are fuppofed to move with one common ve- 

 locity, any pulfes, excited in the ethereal 

 medium^ mafi: overtake them at equal di- 

 ftances ; and therefore the intervals of re- 

 flexion and tranfmifiion, if they arJfe in this 

 manner, as Sir IJaac Newton conjedlures, 

 would be all equal : but, if the red move 

 fwiftcft, the violet fioweft, and the interme- 

 diate colours with intermediate velocities j 

 it is plain, that the fame pulfes mull over- 

 take the violet fooneft, the other colours ia 

 their order, and, laft of all, the red j that 

 is, the intervals of the fits muft be leafi: in 

 violet, and gradually greater in the prifma- 

 tic order ; according to pbfervation. 



42. As the proportion between thefe inter- 

 vals in red and violet can be afiigned by ex- 

 periment, and the proportion of their velo- 

 cities in any med'Mm like ways, by N" 40. 5 

 the velocity of the ethereal pulfes in any me- 

 dium^ and their diftance from one another, 

 pay be thence computed by the following rule: 



« Multiply 

 * l^iv.f. Princip. lib. 2, prop. o. 



