543 OPT 
refraftion there alfo, and fancied he could prove it by af- 
tronomical obfervations. Hence this author concludes, 
that refraction increafes the altitudes of all objefts in the 
heavens; and die firft advanced, that the ftars are fome- 
times fecn above the horizon, by means of refraftion, 
when they are really below it. This obfervation was con¬ 
firmed by Vi tel 1 i o, B. Waltherus, and by the excellent 
obfervations of Tycho Brahe. Alhazen obferved, that 
refraftion contracts the vertical diameters and diitances 
of the heavenly bodies, and that it is the caufe of the 
twinkling of the (tars. But we do not find that either he, 
or his follower Vitellio, CubjeCted it to menfuration. In¬ 
deed, it istoofmall to be determined except by very accu¬ 
rate inflruments ; and therefore we hear little more of it 
til! about the year 1500, whenjgreat attention was paid 
to thefubject by Bernard Walther, Mreftlin, and Tycho 
Brahe. ^ 
In the writings of Roger Bacon, we find the firft dif- 
tinft account of the magnifying power of glades. He 
alfo treats of the appearance of an objeft through a globe, 
and fays that he was the firft who obferved the refraftion 
of rays into it. Bacon was a man of extenlive genius, 
who wrote upon almcft every branch of fcience; yet, in 
optics, fome of the molt abfurd of the opinions of the 
ancients have had the fanCtion of his authority. He be¬ 
lieved that vifual rays proceed from the eye; becaufe 
every thing in nature is qualified to difcharge its proper 
funCtions by its own' powers, in the fame manner as the 
fun and other celellial bodies. In his Specula Mathema- 
tica, he added fome obfervations of little importance on 
the refraCtion of the light of the ftars ; the apparent fize 
of objeCts; the enlargement of the fun and moon in the 
horizon. In his Opus Majus he demonltrates, what Al¬ 
hazen had done before, that if a tranfparent body, inter- 
pofed between the eye and an objeCt, be convex towards 
the eye, the objeCt will appear magnified. 
The great problem concerning the meafure of refrac¬ 
tion was at length folved by Snellius, profedor of mathe¬ 
matics at Leyden. 'It was afterwards explained by Pro- 
feflor Hortenlius before it appeared in the writings of 
Des Cartes, who publiflted it under a different form. But 
before this time, Kepler had publifhed a New Table of An¬ 
gles of RefraCtion, determined by his own experiments, 
for every degree of incidence. Kircher had done the 
fame ; and attempted a theory of refraCtion, on principles 
which, if conducted with precilion, would have led him 
to the law difcovered by Snellius. 
Des Cartes undertook to explain the caufe of refraCtion 
by the refolution of forces. Hence he was obliged to 
fuppofe a paradox, that light paffies with more eafe through 
a denfe medium than through a rare one. The truth of 
this explanation was firft queftioned by M. Fermat, who 
aflerted, contrary to the opinion of Des Cartes, that light 
fuffers more refiftancein water than air, and more in glafs 
than in water; and maintained, that the refiftance of dif¬ 
ferent media with refpeft to light is in proportion to their 
denfities. Leibnitzadopted the fame general idea, upon the 
principle that nature accomplifhes her ends by the (hort- 
eft methods ; and that light, therefore, ought to pafs from 
one point to another, either by the (horteft road, or that in 
which the leaft time is required. 
At a meeting of the Royal Society, Aug. 31, 1664, it 
was found, with a new inftrument prepared for that pur- 
pofe, that the angle of incidence being 40 degrees, that 
of refraCtion is 30. About this time alfo we find the firft 
mention of media not refrading the light in an ex aft pro¬ 
portion to their denfities ; for Mr. Boyle, in a letter to 
Mr. .Oldenburg, dated Nov. 3, 1664, oblerves, that, in 
fpirit of wine, the proportion of the fines of the angles 
of incidence to the fines of the angles of refraCtion was 
nearly the fame as 4 to 3 ; and that, as fpirit of wine oc- 
cafions a greater refraCtion than common water, fo oil of 
turpentine, which is lighter than fpirit of wine, produces 
not only a greater refraCtion than common water, but a 
much greater than fait water. And at a meeting held 
I c s. 
November 9, the fame year, Dr. Hooke mentioned, that 
pure and clear falad-oil produced a much greater refrac¬ 
tion than any liquor which he had tried; the angle of 
refraCtion that anfwered to an angle of incidence of 30 0 
being no lefs than 40 0 30', and the angle of refraCtion 
that anfwered to an angle of incidence of 20 0 being 
2 9 ° 47 '. 
Dr. Hooke made an experiment before the Royal So¬ 
ciety, Feb. n, 1663, which clearly proves that ice re- 
frafts the light lels than water. M. de Ja Hire alfo took 
a good deal of pains to determine whether the refraftive 
power of ice and water were the fame; and he found, as 
Dr. Hooke had done before, that ice refracts lets than 
water. 
By a moft accurate experiment, made in 1698, in which 
a ray of light was tranfmitted through a Torricellian va¬ 
cuum, Mr. Lowthorp found, that the refraftive power 
of air is to that of water as 36 to 34-400. He oblerves, 
that the refraftive power of bodies is not proportioned to 
the denfity, at leaft not to the fpecific gravity, of the re- 
frafting medium. For the refraftive power of glafs to 
that of water is as 55 to 34, whereas its fpecific gravity is 
as 87 to 34 ; that is, the l'quares of their refraftive pow¬ 
ers are very nearly as their refpeftive gravities. And 
there are fome fluids, which, though they are lighter 
than water, yet have a greater power of refraction. Thus 
the refraftive power of fpirit of wine, according to Dr. 
Hooke’s experiment, is to that of water as 36 to 33, and 
its gravity'reciprocally as 33 to 36, or 36£. But the re¬ 
fraftive powers of air and water feem to obferve the Am¬ 
ple direft proportion of their gravities. 
The Royal Academy of Sciences at Paris endeavoured to 
repeat this experiment in 1700; but they did not fucceed ; 
for, as they laid, beams of light pafl’ed through the va¬ 
cuum without buffering any refraCtion. The Royal So¬ 
ciety, being informed of this, ordered Mr. Haukfbee to 
make an inftrument for the purpofe, under the direction 
of Dr. Halley, for the purpofe of repeating the experi¬ 
ment It confifted of a ftrong brafs prifm, two fides of 
which had fockets to receive two plane glades, whereby 
the air in the priftn might either be exhaufted or con- 
denfed. The prifm had alfo a mercurial gage fixed to it, 
to difcover the denfity of the contained air; and turned 
upon its axis, in order to make the refraftions equal on 
each fide when it was fixed to the end of a telefcope. 
The refrafting angle was near 64° 3 and the length of the 
telefcope, having a fine hair in its focus, was about 10 
feet. The event of this accurate experiment was as fol¬ 
lows : Having chofen a proper objeft, whole diftance was 
2588 feet, June 15, 1708, in the morning, the barometer 
being then at 29-7^, and the thermometer at 60, they firft 
exhaufted the prifm, and then, applying it to the tele¬ 
fcope, the horizontal hair in the focus covered a mark on 
the objeft dillinftly feen through the vacuum, the two 
gialfes being equally inclined to the vifual ray. Then 
admitting the air into the prifm, the objeft was leen to 
rife above the hair gradually as the air entered; and, when 
the prifm was full, the hair was obferved to hide a mark 
io| inches below the former mark. After this they ap¬ 
plied the condeniing- engine to the prifm ; and, having 
forced in another atinolphere, fo that the denfity of the 
included air was double to that of the outward, they 
again placed it before the telefcope, and, letting out the 
air, the objeft, which before feemed to rife, appeared gra¬ 
dually to defcend; and the hair at length refted on an ob¬ 
jeft higher than before by the fame interval ot io£ inches. 
They then forced in another atmofphere ; and, upon dis¬ 
charging the condenfed air, the objeft was feen near 21 
inches lower than before. Now the radius in this cafe 
being 2588 feet, 10J incheswill fubtend an angle of 1' 8"; 
and, the angle of incidence of the vifual ray being 32 de¬ 
grees (becaufe the angle of the glafs planes was 64°), it 
follows, from the known laws of refraftion, that, As the 
fine of 39 0 is to that of 31 0 59' 26", (differing from 32 0 by 
34" the half of x' 8",) lo is the fine of any other angle of 
incidence 
