52 Mr. MicuEvuy on the Means of difcovering the 
ratio of 19 to 20, and it was there obferved, that the area re- | 
prefenting the remaining force which would be neceflary to 
generate the velocity 19, was therefore properly reprefented by 
2$-dth parts of the area, that fhould reprefent the force that 
would be neceffary to generate the whole velocity of light, 
when undiminifhed. If then we add 561, the area reprefent- 
ing the force by which the light is accelerated at the furface of 
the glafs, to 361, the area reprefenting the force which would 
have generated the diminifhed velocity of the ftar’s light, the 
{quare root of g22, their fum, will reprefent the velocity of 
the light with the diminithed velocity, after it has entered the 
glafs, And the fquare root of 922 being 30,364, the fines of 
incidence and refraction of fuch light out of air into glafs will 
confequently be as 30,364 to 19, or what is equal to it, as 
31,96 to 20 inftead of 31 to 20, the ratio of the fines of inci- 
dence and refraction, when the light enters the glafs with its 
velocity undiminifhed. 
32. From hence a prifm, with a fmall refracting angle, 
might perhaps be found to be no very inconvenient inftrument 
for this purpofe: for by fuch a prifm, whofe refracting angle 
was of one minute, for inftance, the light with its velocity 
undiminifhed would be turned out of its way 33’, and with 
the diminifhed velocity 35”, 88 nearly, the difference between 
which being almoft 2”. 5 3’”, would be the quantity by which 
the light, whofe velocity was diminifhed, would be turned out. 
of its way more than that whofe velocity was undiminifhed. 
33- Let us now be fuppofed to make ufe of fuch a prifm to 
look at two ftars, under the fame circumftances as. the two ftars. 
in the example above-mentioned, the central one of which 
fhould be large enough to diminifh the velocity of its light one 
twentieth part, whilft the velocity of the light of the other, 
which 
