One of the earliest optical speculations related to the visibility of objects. Pythagoras maintained that objects were seen by particles continually emitted from the sur faces of bodies, and entering the pupil of the eye, while by something which was emitted from the cyc, and which, after meeting with something emitted from the object, was reflected back to the organ of sight. Not withstanding the absurdity of the Platonic doctrine of ocular beams, as they were called, the followers of Plato were acquainted with two very important and funda mental points in the science. They taught that light, from whatever source it was emitted, was propagated in straight lines, and that, when it was reflected from the surface of polished bodies, the angle of refiesion was equal to the angle of incidence.
Although Aristotle reasoned very superficially in his attempt to establish the incorporeal nature of light,—to discover the cause of transparency,—and to explain the bent appearance of a stick held obliquely in the water ; yet he is entitled to considerable credit for his sagacious though vague speculations respecting various optical phenomena. He ascribed the imperfect darkness which exists where the rays of the sun do not shine, to the reflection of light by the atmosphere. He observed pretty accurately the general phenomena of rainbows, halos and parhelia ; and he ascribed the rainbow to the reflection of the sun's rays, from the drops of rain, svhich produced an imperfect image of his disk.
There can be little doubt but that the ancients were at a very early period acquainted with the use of lenses and mirrors, for the purpose of burning bodies placed in their focus. It is probable that the Romans lighted their sacred fires by the aid of concave reflectors ; and notwithstanding the objections which have been urged against the burning apparatus of Archimedes, there is every reason to believe that the Syracusan philosopher destroyed the Roman fleet by means of the heat reflect ed from a number of plain mirrors. See our article Buttxrxc INSTRUMENTS, where this subject has been fully discussed.
The earliest writer on optics was the celebrated Eu clid, who drew up a complete treatise on the subject, w'nich has been handed down to us along with his geo metrical works.f This treatise, which consists of two books, one on optics, containing 60 theorems, and the other on catoptrics, containing 3 theorems, relates to the determination of the apparent magnitude and form of objects from the angle under which they appear, and the apparent place of the images of objects reflected from polished mirrors. In the first part, Euclid en deavours to refute the Pythagorean doctrine of vision ; and he mentions that the visual rays by which objects are seen, form a cone whose apex is in the eye, and whose base is on the object. He proves, in a vety short and perspicuous manner, that the angles of incidence and reflexion are equal, and that both the rays are in a place at right angles to the reflecting surface. He shows
that, in concave mirrors, the objects appear concave, and in convex ones, convex ; that the apparent place of an image formed by a polished mirror is at the place where the reflected ray cuts the line drawn from the ob ject to thc centre IA the sphere ; that o'ojects appear smaller in concave and convex mil rot's, and that in con vex mirrors, objects appear on the side in which they are actually situated. From the general incorrectness by which this treatise is characterized, as well as the number of errors with which it abounds, some authors have been of opinion, that it was not the production of the great geometer, while other suppose that it has re ceived additions and altetations from the hands of suc• ceeding authors. There call be no doubt, however, from the testimony of Proclus and 'rheum that Euclid wrote a work on optics ; though the imperfect nature of the data, ancl the inaccuracy of his reasoning, are very un like what might have been expected from such a skilful geometer, yet, as Montucla has well remarked, there are at all times numerous examples of natural philosophers, whose sagacity and acuteness appear only in their geo metrical writings.
From the time of Euclid to that of Seneca, no pro gress seems to have been made in the study of optics. Seneca observed, that small letters, seen through a bot tle of glass filled with water, appear larger ; and the rea son which he assigned for this effect WaS, " that the eye slides in the water, and cannot lay hold of its object, (quoit acies nostra in humid° labitur, nee apfirchendere cuod vult fideliter potest.) The colour produced by an angular piece ol glass, seems to have excited the atten tion of Seneca. Ile remarked, that such a piece of glass produced all the colours of the rainbow ; but lie regard ed the colour as not real, but as false, like that which appears on the neck of a pigeon, and which changes with the position of the observer. The same false colour, he rernarks, is that of a speculum, which has no colour of its own, but assumes that of any other body. Seneca likewise mentions the magnifying power of concave mirrors; but in his speculations respecting the rainbow, he has rather obscured than improved the conceptions or Aristotle. He considers the circular shape of rain bows and halos, as occasioned by the motion communi cated to the atmosphere, and the vapours it contains, by the impulse of light, the effect of which he supposes must be circular, like the waves produced by a stone falling on the surface of water. He considers the rain bow as the image of the sun reflected from a hollow and moist cloud ; and he supposes that the colours come partly froni the sun, and partly from the cloud.