52 ESSAYS anp OBSERVATIONS 
rent velocities of different colours, we 
may underftand, at leaft in general, 
whence it is, that the intervals of the fits 
may bear a proportion fome way related 
to the fpaces * occupied by the feveral 
. colours in the fpectrum ; an analogy o- 
therways very unaccountable! Since, 
from the velocities of the feveral rays up- 
on which the intervals of the feveral fits 
depend, arife likewife their feveral degrees 
of r2frangibility, which determine the 
{pace occupied by each in the fpectrum. 
AND thus likewife 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 diffe- 
rent magnitude or velocity in the optic 
nerve; by which, according to the laws 
of our conftitution, the ideas of different 
colours may be excited +, in like manner 
as the ideas of different tones arife from 
different vibrations of the air communica- 
* Compare Newt. opt. book 1. part 2. prop. 3. with 
book 2, part 3, prop. 16. 
~ + Newton’s optics, query 13- 
ted 
