TIVE.
Optics, in its more extensive accepta tion, is a mixed mathematical science, which explains the manner in which vi sion is performed in the eye ; treats of sight in general ; gives the reasons ofthe several modifications or alterations which the rays of light undergo in the eye ; and shows why objects appear sometimes greater, sometimes smaller, sometimes more distinct, sometimes more confused, sometimes nearer, and sometimes more remote. In this extensive signification it is considered by Sir Isaac Newton, in his Optics. Indeed optics make a consider able branch of natural philosophy ; both as it explains the laws of nature, accord ing to which vision is performed, and as it accounts for abundance of physical phe nomena, otherwise inexplicable.
The reflection of the rays of light is, indeed, an occurrence too frequent and obvious to have escaped the notice even of the earliest observers; a river or some other piece of water was probably the first mirror ; its effect was afterward imitated by metallic mirrors : hence Was discovered the equality of the angles of incidence and reflection. It was known at an early period that an oar, or other straight piece of wood, partially immers ed in water, no longer appeared straight, yet ages after this elapsed before any at tempts were made to ascertain the rela tion between the angles of incidence and refraction. Empedocles was the first person on record that wrote systemati cally on light ; and Euclid composed a treatise on the ancient optics and catop tries ; dioptrics being less known to the ancients, though it was not quite unno ,ticed by them, for among the phenomena at the beginning of that work, Euclid re marks the effect of bringing an object in to view, by refraction, in the bottom of a vessel, by pouring water into it, which could not be seen over the edge of the vessel before the water was poured in ; and other authors speak of the then known effects of glass globes, &c. both as burning glasses, and as to bodies seen through them. Euclid's work, the genu ineness of which has been doubted, is chiefly on catoptrics, or reflected rays ; in which lie shows the chief properties of them in plane, convex, and concave sur faces, in his usual geometrical manner, beginning with that concerning the equa lity of the angles of incidence and re flection, which he demonstrates ; and in the last proposition, showing the effect of a concave speculum, as a burning glass, when exposed to the rays of the sun.
The effects of burning glasses, both by refraction and reflection, are noticed by several others of the ancients, and it has been thought that the Romans had a me thod of lighting their sacred fire by some such means. Aristophanes, in one of his comedies, introduces a person as making use of a globe filled with water to cancel a bond that was against him, by thus melt ing the wax of the seal. If we give cre dit to what some ancient historians are said to have written concerning the ex ploits of Archimedes, we shall be induced to think that he constructed some very powerful burning mirrors. It is even al lowed that this eminent geometrician wrote a treatise on the subject of them, though it be not now extant ; as also con cerning the appearance of a ring or cir cle under water, and therefore could not have been ignorant of the common phe nomena of refraction. We find many questions concerning optical appearances in the works of Aristotle. This author was also sensible that it is the reflection of light from the atmosphere which pre vents 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 produced, the colour only being ex hibited, and not his proper figure. The ancients were not only acquainted with the more ordinary appearances of refrac tion, but knew also the production of co lours by refracted light. Seneca says, that when the light of the sun shines through an angular piece of glass, it shows all the colours of the rainbow. 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.