appear higher in the morning before funrife, and aifo late 
in the evening, than at noon, in a clear day, by feveral 
minutes. In one cafe the elevations of the fame hill dif¬ 
fered more than thirty minutes. 
M. Euler considered the refractive power of the atrno- 
fphere, as affeided by different degrees of heat and elafti- 
city ; in which he fhows, that its refractive power to a 
contiderable diftance from the zenith, is fufficiently near 
the proportion of the tangent of that diftance, and that 
the law of refraction follows the direct ratio of the dif¬ 
ference marked by the thermometer; but, when ftars are 
in the horizon, the changes are in a ratio fomewhat greater 
than this, more efpecially on account of the variation in 
the heat. 
As the denfity of the atmofphere varies with its alti¬ 
tude, and as the irregular curvature of the earth caufes 
a cor.ftant change in the inclination of the ftrata through 
■which any ray of light paffes to the eye, the refraction 
cannot be obtained from the denfity of the atmofphere, 
and the angular direction of the refracted ray. By com¬ 
paring aftronomical with •'meteorological obfervations, 
however, the celebrated M. La Place has given a complete 
folution of this very important problem. 
The phenomena known by the names of mirage, loom- 
ing, and fata morgana, have been traced to irregularities 
of refradions arifing from accidental changes in the tem¬ 
perature of the atmofphere. From the rarefra&ion of 
the air near the furface of water, buildings, or the earth 
itfelf, a diftant objeCt feen througdi this rarefied air fome- 
times appears deprefifed inftead of railed by refraftion ; at 
other times, it appears both elevated and deprefied ; fo 
that the objeft feems double, and fometimes triple, one 
of the images being in an inverted pofition. This fub- 
jeCt is much indebted to the refearches of the ingenious 
Hr. Wollafton, who has imitated thefe natural pheno¬ 
mena by viewing objefts through the rarefied air conti¬ 
guous to a red-hot poker, or through a faline or faccha- 
rine folution with water and fpirit of wine floating upon 
its furface. This branch of optics has alfo been well il- 
iuftrated by Mr. Vince and Mr. Huddart. 
Difcoveries concerning the Rcflettion cf Light. 
The followers of Plato were acquainted with the equa¬ 
lity between the angles of incidence and refletflion ; and 
it is probable that they discovered this, by obfervinga ray 
of the fun reflected from Handing water, or fome other po- 
liflied body ; or from attending to the images of objects 
reflected by fuch furfaces. If philofophers paid any at¬ 
tention to this phenomenon, they could r.ot but perceive, 
that, if the ray fell nearly perpendicular upon fuch a fur¬ 
face, it was reflected near the perpendicular; and, if it fell 
obliquely, it was reflected obliquely; and obfervations 
upon thefe angles, the molt rude and imperfeft, could 
not fail to convince them of their equality, and that the 
incident and refieCIed rays were in the fame plane. 
Ariftotle was fenfible that it is the reflection of light 
from the atmofphere which prevents total darknefs after 
the fun fets, and in places where he does not Shine in the 
day-time. He was alfo of opinion, that rainbows, halos, 
and mock funs, were occasioned by the reflection of the 
fun-beams in different circumftances, by which an imper- 
feCt image of his body was produced, the colour only 
being exhibited, and not his proper figure. The image, 
lie fays, is not Single, as in a mirror ; for each drop of 
rain is too fmall to refleCt a vifible image, but the con¬ 
junction of all the images is vifible. 
Without inquiring any farther into the nature of light 
or vifion, the ancient geometers contented themfelves 
with deducing a fyftem of optics from two faCts; the 
rectilineal progrefs of light, and the equality of the an¬ 
gles of incidence and reflection. The treatife of optics 
aferibed to Euclid is employed in determining the appa¬ 
rent fize and figure of objeCts, from the angle which they 
fubtendat the eye, and the apparent place of the image 
of an objeCt reflected from a polished mirror. This place 
he fixes at the point where the reflected ray meets a per¬ 
pendicular to the mirror drawn through the objeCt. 
It was Kepler who firft discovered, that the apparent 
places of 1 objeCts, feen by reflecting mirrors, depended 
upon the angle which the rays of light, iffuing from the 
extreme part of an object, make with one another after 
reflection. 
Mr. Boyle made fome curious obfervations concerning 
the reflecting powers of differently-coloured fubftances. 
In order to Show that fnow Shines by a borrowed, and 
not by a native, light, he placed.a quantity of it in a 
room, from which all foreign light was excluded, and 
found that it was completely invisible. To try whether 
white bodies refleCt more light than others, he held a Sheet 
of white paper in a funbeam admitted into a darkened 
room ; and obferved that it reflected much more light 
than a paper of any other colour, a considerable part of 
the room being enlightened by it. To Show that white 
bodies refleCt the rays outwards, he adds, that common 
burning-glafi'es require a long time to burn or difcolour 
white paper ; that the image of the fun was not fo well 
. defined upon white paper as upon black; that, when lie 
put ink upon the paper, the moifture would be quickly 
dried up, and the paper, which he could not burn before, 
would prefently take fire ; and that, by exposing his hand 
to the fun, with a thin black glove upon it, it would be 
Suddenly and more considerably heated, than if he held 
his naked hand to the rays, or put on a glove of thin 
white leather. 
Dr. Hooke was the firft who obferved the beautiful co¬ 
lours that appear in thin plates of Mufcovy-glafs. With 
a microfcope he could perceive that thefe colours were 
ranged in rings furrounding the white fpecks or flaws in 
this thin fubftance, that the order of the colours was the 
very fame as in the rainbow, and that they were often re¬ 
peated ten times. But the colours were difpofed as in 
the outer rainbow. Some of them alfo were much 
brighter than others, and fomeofthem very much broader. 
He alfo obferved, that if there was a part where the co¬ 
lours were very broad, and confpicuous to the naked eye, 
they might be made, by preffmg the part with the finger, 
to change places, and move from one part to another. 
Laftly, he obferved, that, if great care be ufed, this fub¬ 
ftance may fplitinto plates of § or of an inch in diame¬ 
ter, each of which will appear through a microltope to 
be uniformly adorned with fome one vivid colour ; and 
that thefe prates will be found upon examination to be of 
the fame thicknefs throughout. 
A phenomenon fimiiar to this was noticed by lord 
B rereton, who, at a meeting of the Royal Society in 1666, 
produced fome pieces of glafs taken out of a church- 
window, both on the north and on the fouth fide of it ; 
they were all eaten in by the air ; but the piece taken 
from the fouth fide had fome colours like thofe of the 
rainbow upon it, which the others on the north fide had 
not. It cannot be doubted, but that in all thefe cafes, 
the glafs is divided into thin plates, which exhibit co¬ 
lours, upon the fame principle with thofe which Dr. Hooke 
obferved in the bubble of foap and wafer, and in the 
thin plate of glafs, which we fhall find more fully ex¬ 
plained by fir Ifaac Newton. 
The enquiries of M. Bouguer concerning the reflec¬ 
tion of light are worthy of particular notice. They are 
fully detailed in Ills Traite cl'Optique, a pollhumous work 
publiflied by LaCaiilein 1760. In order to compare dif¬ 
ferent degrees of light, he always contrived to place the 
radiant bodies', or other bodies illuminated by them, in 
fuch a manner that he could view them diftinftly at the 
fame time ; and he either varied the diftances of thefe 
bodies, or modified their light in fome other way, till be 
could perceive no difference between them. Then, con- 
fidering their different diftances, or the other circum¬ 
ftances by which their light was affe&ed, he calculated 
