OPTICS. 
far from being the cause of the heavenly bo- 
dies appearing larger near the horizon, that 
it would make them appear less ; observing 
that two stars appear nearer together in the 
horizon, than near the meridian. This phe- 
nomenon he ranks among optical decep- 
tions. We judge of distance, he says, by 
comparing the angle under which objects 
appear, with their supposed distance; so 
that if these angles be nearly equal, and 
the distance of one object be conceived 
greater than that of the other, this will be 
imagined to be the larger. And he further 
observes, that tlie sky near the horizon is 
always imagined to be further from us than 
any other part of the concave surface. 
In the writings of Alhazen, too, we find 
the first distinct account of the magnifying 
power of glasses, and it is not improbable 
that his wj itings on this head gave rise to 
the useful invention of spectacles ; for he 
says, that if an object be applied close to 
the base of the larger segment of a sphere 
of glass, it will appear magnified. He also 
treats of the appearance of an object through 
a globe, and says that he was the first who 
observed the refraction of rays into it. 
In 1270, Vitellio, a native of Poland, 
published a treatise on optics, containing 
all that was valuable in Alhazen, and di- 
gested in a better manner. He observes, 
that light is always lost by refraction, which 
makes objects appear less luminous. He 
gave a table of the results of his experi- 
ments on the refractive powers of air, wa- 
ter, and glass, corresponding to different 
angles of incidence. He ascribes the twink- 
ling of the stars to the motion of the air in 
which the light is refracted ; and he illus- 
trates this hypothesis by observing, that 
they twinkle still more when viewed in wa- 
ter put in motion. He also shows, that re- 
fraction is necessary as well as reflection, 
to form the rainbow ; because the body 
which the rays fall upon is a transparent 
substance, at the surface of which one part 
of the light is always reflected, and another 
refracted. And he makes some ingenious 
attempts to explain refraction, or to ascer- 
tain the law of it. He also considers the 
foci of glass spheres, and the apparent size 
of objects seen through them, though with 
but little accuracy. See Refraction, 
Contemporary with Vitellio was Roger 
Bacon, a man of very extensive genius, who 
wrote upon almost every branch of science; 
though it is thought his improvements in 
optics were not carried far beyond those of 
Alhazen and Vitellio : to him, however, has 
been attributed the invention of the Magic 
Lantern, which see. 
One of the next who distinguished him- 
self in this way wms IMaurolycus, teacher 
of mathematics at Messina. In a treatise, 
“ De Lumine et Umbra,” published in 157,5, 
he demonstrates, that the crystalline hu- 
mour of the eye is a lens that collects the 
rays of light issuing from the objects, and 
throws them upon the retina, where the fo- 
cus of each pencil is. From this principle 
he discovered the reason why some people 
are short-sighted, and others long-sighted ; 
also why the former are relieved by con- 
cave glasses, and th,e others by convex 
ones. 
Contemporary with Maurolycus was John 
Baptista Porta, of Naples. He discovered 
the camera obscura, which throws consider- 
able light on the nature of vision. His 
house was the constant resort of all the in- 
genious persons at Naples, whom he formed 
into what he called An Academy of Secrets, 
each member being obliged to contribute 
something that was not generally known, 
and might be useful. By this means he was 
furnished with materials for his “ Magia 
Naturalis,” which contains his account of 
the camera obscura, and the first edition of 
which was published, as he informs us, when 
he was not quite fifteen years old. He also 
gave the first hint of the magic lantern, 
which Kircher afterwards followed and im- 
proved, His experiments with the camera 
obscura convinced him, that vision is per- 
formed by the intromission of something 
into the eye, and not by visual rays pro- 
ceeding from it, as had been formerly ima- 
gined ; and he was the first who fully satis- 
fied himself and others upon this subject. 
He justly considered the eye as a camera 
obscura, and the pupil the hole in the win- 
dow-shutter ; but he was mistaken in sup- 
posing that the crystalline humour corre- 
sponds to the wall which receives the ima- 
ges; nor was it discovered till the year 
1604, that this office is performed by the 
retina. He made a variety of just remarks 
concerning vision, and particularly ex- 
plained several cases in which we imagine 
things to be without the eye, when the ap- 
pearances are occasioned by some affection 
of the eye itself, or by some motion with- 
in the eye. He remarked also, that, in 
certain circumstances, vision will be assisted 
by convex or concave glasses ; and be seem* 
even to have made some small advances to- 
