PHYSICAL AND LITERARY. 47 



the velocity of the ethereal pulfes above that 

 of the given colour: but, even upon the fup- 

 pofition of the truth of our theory, an exzdi 

 coincidence between calculation and experi- 

 ment is not to be expeded till the velocities 

 of the rays be more accurately determined. 

 ,■•46. Upon the hypothefis of the different 

 velocities of different colours, we may under-* 

 ftand, at leafi in general, whence it is, that 

 the intervals of the fits may bear a propor- 

 tion fome way related to the fpaces* occu- 

 pied by the feveral colours in the fpedrum j 

 an analogy oth^rways very unaccountable 1 

 Since, from the velocities of the feveral rays 

 upon which the intervals of the feveral fits 

 depend, arife likeways their feveral degrees of 

 refrangibility, which determine the fpace 

 occupied by each in the fpedtrum. 



Aj^d thus likeways we may conceive, how 

 the different rays are qualified to produce 

 different fenfations in the mind : for, having 

 different degrees of impulfive force, they 

 may caufe vibrations of different magnitude 

 or velocity in the optic nerve ; by which, 



according 



* Compare iV^w/. Opt. b. i. part 2. prop. 3. with b,u. 

 part 3, prop. i6th. 



