$4. ESSAYS anp OBSERVATIONS 
any number of tranfparent bodies any 
how difpofed *, _ 
Ir is impoffible therefore to know, 
whether an alteration of the fwiftnefs, 
with which a homogeneal ray ftrikes the 
retina, would alter its colour ; I mean the 
fenfation of colour produced by it in the 
mind; Since itis impoflible to alter, at 
pleafure, the denfity of that fluid which 
determines its final velocity. 
One may diftinguith two different ef- 
fects of the refractive power on the rays 
of light, viz. the change of direction and 
change of velocity. Sir Ifaac Newton 
has proved, with refpect to the firft, That 
itis different in the differently-coloured 
rays, and ofa determined degree in each: 
He has further proved, that refraction, 
confidered in its firlt effeA, does not 
* Here it is proper to obferve, that the hypothelis 
which fuppofes the intervals of the fits to be determined by 
the velocity of the ray, agrees well with a remarkable ob- 
fervation of Sir Ifaac Newton (Optics, book 2. part 1. obf. 
21): viz that thefe intervals in any medium, at a given 
angle of iacidence. are of a given magnitude, without re- 
gardto the denfity of the furrounding medium. 
change 
