These are the only mathematical treatises on optics of any consideration which the an cients have transmitted to us, but many metaphysical speculations on light and vision are to be found in the writings of the philosophers. Aristotle defined light much as he had defined motion; the act or energy of a transparent body, in as much as it is. transparent.
The reason light an act of a transparent body is, that, though a body may be transparent in power or capacity, it does not become actually transparent but by means of light.. Light brings the transparency into action ; it is, therefore, the act of a transpa rent body. In such miserable puerilities did the genius of this great man exhaust itself, owing to the unfortunate direction in which his researches were carried on.
In his farther speculations concerning light, he denied it to be a substance ; and his argument contains a singular mixture of the ingenious and the absurd. The time, he says, in which light spreads from one place to another is infinitely small, so that light has a velocity which is .infinitely great. Now, bodies move with a velocity inversely as the quantities which they contain ; light,. therefore, cannot contain any mat.
ter, that is, it cannot be material. That the velocity of light was infinitely seemed. to him to fellow from this, that its progress, estimated either in the direction of north and south, or of east and west, appeared to be instantaneous. In the opinion of the Platonists, and of the greater part of the aneients, vision was performed by means of certain rays which proceeded from the eye to the object, though they did not become thd instruments of _conveying sensations to the mind, but in consequence of the presence of light. In this theory, we can now see nothing but a rude and hasty attempt to assimi late the sense of sight to that of touch, without inquiring sufficiently into the particular characters of either. • Epicurus, and the philosophers of his school, as we learn from Lucretius, entertained more correct notions of vision, though they were still far from the truth. They conceiv ed to be performed in consequence of certain simulacra, or images continually thrown off from the surfaces of bodies, and entering the eye. This was the substitute in their philosophy for rays of light, and had at least the merit of representing that _which ' is the medium of vision, or which forms the communication between the eye" and nal objects, as something proceeding from the latter. The idea of simulacra, or spectra, flying off continually from the surfaces of bodies, and entering the eye, was perhaps as _ near an approach to the true theory of vision as could be made before the structure of the eye was understood.
In the arts connected with optics, the ancients had made some progress. They were sufficiently acquainted with the laws of reflection to construct mirrors both plane and spherical. They made them also conical ; and it appears from Plutarch, that the fire of Vesta, when extinguished, was not permitted to be rekindled but by the rays of the sun, which were condensed by a conical speculum of copper. The mirrors with which Archimedes set fire to the Roman pales have been subjects of much discussion, and the fact was long disbelieved, on the ground of being physically impossible. The experiments of Kircher and Buffon showed that this impossibility was entirely imaginary, and that the effect ascribed to the specula of the Greek geometer might be produced without much difficulty. There remains now no doubt of their reality. A passage from Aristophanes 4 gives reason to believe that, in his time, lenses of glass were used for burning, by col lecting the rays of the sun ; but in a matter that concerns the history of science, the au thority of a comic poet and a satirist would net deserve much attention, if it were not con firmed by more sober testimony. Pliny, speaking of rock crystal, ' says that a globe or ball of that substance was sometimes used by the physicians for collecting the rays of the sun, in order to perform the operation of cautery. In another passage, he mentions the power of a glass globe filled with water, to produce a strong heat when exposed to the rays of the sun, and expresses his surprise that the water itself should all the while remain quite cold..
With respect to the power of glasses to magnify objects seen through them, or to ren der such objects more distinct, the ancients appear to have observed ill, and to have rea soned worse. " Literve quamvis minutse et obscurse per vitreous pilaw aqua plenam majores clarioresque cernuntur. Sidera amphora per nubem adspicienti videntar quiz acies rostra in hunsido labitur, nee apprekendere quod twit fiddlier point." This passage, and the speculations concerning the rainbow in the same place, when they are considered as containing the opinions of some of the most able and best informed men of antiquity, must be admitted to mark, in a very striking. manner, the infancy of the phy sical sciences.