6.2 ESSAYS AND OBSERVATIONS 



QuER. III. Is light emitted with the fame 

 velocity, in whatever medium the luminous 

 body be placed ? Or, is it not rather emitted 

 with greater velocity in denfer mediums^ and 

 that in proportion to their refraftive powers? 

 The iame argument from whence we 

 gather in general the equal velocity of light 

 emitted by all forts of luminous bodies, feems 

 to prove the truth of the latter fuppofition. 

 For, fince rays of any one colour, from the 

 fun and a candle, for inftance, are equally 

 refraded by a furface of glafs or water, we 

 ITiay conclude, that their velocities in air are 

 equal. Wherefore, if the denfity of the 

 fun's atmofpbere, contiguous to his furface, 

 be different from the denfity of our lower 

 air, as may be fafely prefumed, his rays 

 muil: have been emitted with more or lefs 

 velocity than that of the candle ; otherways, 

 they could not have the fame velocity after- 

 wards in any common medium : for, the ve- 

 locity with which any ray is emitted, is, by 

 the laws of refraftion, to its velocity in any 

 given medium., as the fine of refradion to the 

 line of incidence, when a ray pafiTes from 



the 



