Royal Society. 
411 
By the refraaive power of a pellucid body, Mr. Loivtborp 
means that property in it, whereby the oblique rays of light are 
diverted from their direft courfe, and which is meafured by the 
proportional differences, always oblerved, between the fines of 
the angles of incidence and emcrfion : This property is not always 
proportional to the denfity, at leaft not to the gravity, of the re- 
frading medium 5 for the refraftive power of glals to that of wa> 
ter is as 55 to 54, whereas its gravity is as 87 to 34 j that is, the 
fquares of their refradive powers are very nearly as their ref- 
peclive gravities 5 and there are fome fluids, which, tho' lighter 
than water, yet have a greater power of refraction 5 thus the re- 
fradive power of fpirit of wine, according to Dr. Hook'^ expe- 
riment, is to that of water as 35 to 235 and its gravity recipro- 
cally as 23 to 36, or 3 5tj but the refra^ive powers ot air and 
water feem to obferve the fimple proportion of their gravities 
direaiy, as they are compared in the following table j the num- 
bers there expreffing the refraction of water are taken from the 
mean of nine experiments made at fo many leveral angles of inci- 
dence, Jan, 25th, 1647, by the ingenious Mr. Gafcoigne;, (the 
firft inventor of the micrometer, and the ways of meaiuring an- 
gles by telefcopes ) and thofe of air are produced by the preceed- 
ing experiment : 
Water Air 
The (affumed) fines of the angles of inci-? 
dence on the void, from — — ■ — ^ 
The fines of the correlpondent angles of^ 
emerfion out of — — — — — i 
The refractive power of — — - — — 
The fpecific gravity (if as 900 to i at the? ^^^^^ 
time of the experiment) of — — • — ^ ^ 
Or (if as 850 to i) of -^ — — . — 
lOOOOO 
134400 
34400 
34400 
1 00000 
10003^ 
3(J 
40 
From hence it feems very probable, that their refpe^ive den- 
fitics and refraCtive powers are in a jull fimple proportion 3 ^^d it 
this fhould be confirmed by fucceedmg experiments, made at dif- 
ferent angles of incidence, and with cylinders continuing ex- 
haufted thro' leveral changes of the air, it would be more than 
probable that the refraftive powers of the atmofphere are every 
where, and at all heights above the earth, proportional to its den- 
fity and expanfionj and then it would be no difficult matter to 
trace the light thro' it, {o as to terminate the fhadow of the 
€arth, and toosther with proper expedients for meafuring the 
JF f f a quantity 
