PHYSICAL ann ‘LITERARY. 53 
‘ted to the auditory-organ. It has been 
faid, that the different fenfations excited 
in the mind cannot arife from the diffe- 
rent force of the particles of light; fince 
the colour of homogeneal rays is not al- 
tered by pafling through different media, 
though their velocity be thereby always — 
increafed or diminifhed *. But it ought 
to be confidered, that every ray, as it mult 
pafs at lait through the humours of the 
eye in order to vifion, falls upon the refz- 
na with one given velocity, whatever num- | 
ber of refraQtions it has previoufly under- 
gone: For the velocity of any ray in any 
one medinm being, to its velocity in any 
other medium, in a conftant proportion, 
viz. the inverfe of the fines of incidence 
and refraétion, when a ray pafles from 
the.one into the other; it is manifeft, that 
each ray muft have a certain determined 
velocity i in any given medium, which can- 
not be either increafed or diminifhed by 
making the ray pafs previoufly through 
a 
* Mufichenbroeck elementa phyfices, § 1161, 
any 
