OPTICS. 
Seen through them. Euclid’s work, the 
genuineness of which has been doubted, is 
chielly on catoptrics, or reflected rays ; in 
which he shews the chief properties of 
them in plane, convex, and concave sur- 
faces, in his usual geometrical manner, be- 
ginning with that concerning the equality 
of the angles of incidence and reflection, 
which he desnonstrates ; and in the last pro- 
position, showing the effect of a concave 
speculum, as a burning glass, when exposed 
to the rays of tlie sun. 
The effects of burning glasses, botli by 
refraction and reflection, are noticed by se- 
veral others of the ancients, and it has been 
thought that the Romans had a method ot 
lighting their sacred fire by some such 
means. Aristophanes, in one of his come- 
dies, introduces a person as making use of 
a globe filled with water to cancel a bond 
that was against him, by thus melting the 
wax of the seal. If we give credit to what 
some ancient historians arc said to have 
written concerning the ex|)loits of Archi- 
medes, we shall be induced to think that 
he constructed some very powerful burning 
mirrors. It is even allowed that this emi- 
nent geometrician wrote a treatise on the 
subject of them, though it be not now ex- 
tant ; as also concerning the appearance of 
a ring or circle under water, and therefore 
could not have been ignorant of the com- 
mon phenomena of refraction. We find 
many questions concerning optical appear- 
ances in the wwrks of Aristotle. Uhis au- 
thor was also sensible tliat it is the reflec- 
tion of light from the atmosphere which 
prevents total darkness after the sun sets, 
and in places where he does not shine in 
the day time. He was also of opinion, that 
rainbows, halos, and mock suns were all 
occasioned by the reflection of the sun- 
beams in different circumstances, by which 
an imperfect image of his body was pro- 
duced, the colour only being exhibited, and 
not his proper figure. The ancients were 
not only acquainted with the more ordinary 
appearances of refraction, but knew also 
the production of colours by refracted 
light. Seneca says, that when the light of 
the sun shines through an angular piece of 
glass, it shows all the colours ot the rain- 
bow. These colours, however, he says, 
are false, such as- are seen in a pigeon's 
neck when it changes its position ; and of 
the same nature he says is a speculum, 
which, without having any colour of its 
own, assumes that of any other body. 
It appears also, that the ancients were 
not unacquainted with tlie magnifying power 
of glass globes filled with water, though 
tliey probably knew nothing of the reason 
of this power ; and it is supposed that the 
ancient engravers made use ot a glass globe 
filled with water to magnify their figures, 
that they might work to more advantage. 
Ptolemy, about the middle of the second 
century, wrote a considerable treatise on 
optics. The work is lost ; but from the ac- 
counts of others it appears tliat he tliere 
treated of astronomical refractions. Ihe 
first astronomers were not aware that tlie 
intervals between stars appear less when 
near the horizon than in the meridian ; and 
on tliis account tliey must have been much 
embarrassed in their observations ; but it is 
evident that Ptolemy was aware of this cir- 
cumstance by the caution which he gives 
to allow something for it, whenever recourse 
is had to ancient observations. This philo- 
sopher also advances a very remote hypo- 
thesis to account for the remarkably great 
apparent size- of the sun and moon when 
seen near the horizon. The mind, he says, 
judges of the size of objects by means of a 
preconceived idea of their distance from us ; 
and this distance is fancied to be greater 
when a number of objects are interposed 
between the eye and the body we are view- 
ing, which is the case when we see the hea- 
venly bodies near the horizon. In his Al- 
magest, however, he ascribes tliis appear- 
ance to a refraction of the rays by vapours, 
whicli actually enlarge the angle under 
which the luminaries appear, just as the an- 
gle is enlarged by which an object is seen 
from under water. See Ptolemy. 
Alhazen, an Arabian writer, was the next 
author of consequence, who wrote about the 
year 1100. Alhazen made many experi- 
ments on refraction, at the surface between 
air and water, air and glass, and water and 
glass; and hence he deduced several pro- 
perties of atmospherical refraction, such as, 
that it increases tlie altitudes of all objects 
in the heavens ; and he first advanced that 
the stars are sometimes seen above the ho- 
rizon by means of refraction, when they are 
really below it ; which observation was con- 
firmed by Vitellio, Walther, and especially 
by the observations of Tycho Brahe. Al- 
hazen observed, that refraction contracts 
the diameters and distances of the heavenly 
bodies, and that it is the cause of the twink- 
ling of the stars. This refractive power he 
ascribed, not to the vapours contained in 
tlie air, but to its different degrees of trans- 
parency. And it was his opinion, tliat so 
