4« ESSAYS AND OBSERVATIONS 



As light moves from the fun to us, by Dr. 



Bradley^ lateft computation *, in &', 12", 

 the pulfes of the ethereal fluid will be pro- 

 pagated thro' the fame fpace in L', i". 



44. The dijflance between the ethereal 

 pulfes, is, to "f- the interval of the fits in red, 

 as the difference between the velocity of the 

 ethereal pulfes and that of red light is to the 

 velocity of red light ; that interval, there- 

 fore, is not much more than —^ of the in-^ 

 terval of the fits in red, and therefore does 



not much exceed TTr'TTT' o^^" i"^^ +• 



45. The velocity of the ethereal pulfes^ 

 being determined, as above, from the inter- 

 vals of the fits in the two extreme colours, 

 as found by experiment, the intervals of the 

 fits in the fix intermediate rays may be calcuf 

 l;ated from theory i for the interval in any one 

 colour muft be, to that in red, as a produdt 

 under the velocity of the given colour and 

 the excefs of the velocity of the ethereal 

 pulfes above that of red, is to a produd^ 

 under the velocity of red and the excefs of 



the 



• ■ * See Eavtci Abrldg. tranfafl. vol. vi. p. 157. 



■f See note * to foregoing page. 

 ' X See the table of the thicknefs of coloured plates in 

 JVifw.'. Ope. par: s. book z. 



