OPTICS. That branch of natural philoso phy which treats of the nature of light and colours, or of the general doctrine of vision. It is distinguished Into three kinds : namely, optics, properly so called, which treats of di rect vision ; catoptrics, which treats of reflect ed vision, or that which is performed by means of rays reflected from speculums or mir rors ; and dioptrics, which treats of refracted vision, or that which is performed by means of rays refracted or turned out of their course by passing through mediums of different densines,chiely through glasses or lenses. OPTICS, HISTORY OF. The properties of light naturally attracted, at an early period, the attention of philosophers who made nature their study. Empedocles, who flourished up wards of four hundred years before Christ, is said to have written a treatise on light ; and the works of Aristotle present us with a num ber of questions and observations concerning optical appearances. This philosopher was aware that it is the reflection of the light from the atmosphere which prevents total darkness after the sun sets, and in those places where lie does not shine in the daytime. He also considered that rainbows, halos, and mock suns, were all occasioned bythe reflection of the sunbeams in different circumstances, by which an imperfect image of his body was produced, the colour only, and not his proper figure, being exhibited. Euclid wrote a treatise on optics and catoptrics, in which he shows the chief properties of, reflected rays in plane, convex, and concave surfaces, in a geo metrical manner, beginning with that con cerning the equality of the angles of inci dence and reflection. He also takes some notice of dioptrics, and remarks on the effect of refraction in regard to art object at the bottom of a vessel, which, when water is poured in, is brought to view, that was not to be seen above the edge of the vessel before the water was poured in. As to the effect of burning glasses, both by reflection and refrac tion, this is noticed not only by Euclid, but by many other of the ancients ; and, if we give credit to historians, the exploits perform ed by Archimedes in setting fire to the vessel.
of the Romans before Syracuse by means of burning mirrors, prove that his practical knowledge exceeded that of modern times. There is no doubt that he wrote a treatise on burning glasses, as also concerning the ap pearance of a ring or circle under the water, which shows that this phenomenon had not escaped his notice. The ancients were also acquainted with the production of colours by means of refracted light. Seneca observes that when the light of the sun shines through an angular piece of glass, it shows all the colours of the rainbow ; also that the colours seen in a pigeon's neck, when it changes its position, are the effect of refraction, and on the same principle that a speculum, not having any colour of its own, will assume that of any other body.
Besides, the ancients were not unacquainted with the magnifying power of glass globes filled with water, for the ancient engravers used to employ such a glass globe, in order, as is supposed, to magnify the figures, that they might execute their work with more cor recuiess. Ptolemy, who wrote a considerable treatise on optics, was well acquainted with the refraction of light, and determined the ratio of the angles of refraction, as compared with that of the angles of incidence, with such accuracy, that there is but a trifling dif ference between the results of his observations and those of Newton ; not more than might arise from his having used glass and water of specific gravities something different from those employed by Newton. It appears also from this work of Ptolemy, as also from his Almagest, that he employed his knowledge of optics in his astronomical observations, for he was fully aware that refraction decreases from the horizon to the zenith, and that, by means of this refraction, the intervals between the stars appear less when near tho horizon than in the meridian. He also accounts for the remarkably great apparent size of the sun and moon when seen near the horizon, by as cribing the appearance to the refraction of the rays by vapours, which actually enlarge the angle under which the luminaries appear, just as the angle is enlarged by which an ob ject is seen from under water.