Cleornecles, who flourished about 50 years after Christ, has given, at the end of his Cyclical Theory of Meteors, a name then applied to the stars, a very detailed account of the manner in which ViS.1011 supposed to be per formed by those who rejected the rational theory of Py thagoras. They supposed that the visual ray set out from the eye, and moved in a straight line till it encoun tered some obstacle. If the obstacle was impenetrable, it reflected the visual ray, forming equal angles on each side of the perpendicular. In this new direction it seiz ed the objct which was found in its way, and in this manner it rendered it visible. If the body was pene trable, the visual ray passed through it, and assumed the tint of the penetrable medium, which they called dia phanous. If the visual ray passed from a diaphanous medium more rarc, such as air, into a diaphanous me dium mote dense, such as water or glass, it inclined to wards the perpendicular, and seized the object which it met in this new direction. lf, on the contrary', the se cond diaphanous body was rarer than the first, the visual ray inclined towards the other side, or towards the pro longation of the perpendicular, that is, instead of ap proaching the first perpendicular, it receded from it.* The most celebrated of all the ancient writers on Op tics was Claudius Ptolemya the illustrious astronomer of Alexandria, Wil0 flourished about the middle of the second century. His work, which existed in the time of Roger BaC011, has been long supposed to be lost, and it was known, only from those authors who had seen it, that it treated fully on astronomical refractions, and con tained a rational explanation of the enlargement of the horizontal moon. NIontucla informs us, that in the cata logue of the Bodleian library at Oxford, p. 300, there is a wotk, entitled, Opticorum Scrmones yuinque ex .4rabico Latine versi; but though he mentions the circumstance as a proof that the work of Ptolemy is not lost, no person, so tar as we know, has though( of looking into this valuable and interesting IMS.$ In his Exposition of the System of the NVorld, M. Laplace ex pressly states, that there is a similar MS. in the Royal Library at Paris, and in consequence of this, M. Hum boldt was induced to examine the MS. and to communi cate it to the celebrated French astronomer, Al. le Cite valjer Delambrc, who has published a full analysis of it in the Connaissance des Tenzs for 1816.
The optics of Ptolemy consists of Five books. The First book is wanting, but from the recapitulation of it at the beginning of the second, it appears to have contained a dissertation on the relations between light and vision, founded on the idea that the visual rays issue from the eye. In the Second book, he shows that WC see bettcr with two eyes than with one, and that the object is not seen in the same place with one eye as with two. Vision, lle says, is single, if the two axes of the pyramids of the visual rays are directed in the same manner on the ob ject, hut becomes double, if the axes are not directed in a similar manner, and if the distance is a little less than the distance between the eyes. He next proceeds to find geometrically the circumstances which produce single or double images. He ascribes imperfection of sight in old men to a want of' the visual virtue, which, like the other faculties, decays with the approach of age; and he states, that these who have concave eyes see at a less distance than those who have not such eyes. Rapidity
of motion, he asserts, confounds the colours on a wheel. If the colour is in the direction of a ray, the wheel will appear entirely of this colour, and if different colours are at different distances from the centre, these will ap pear on the wheel as so many concentric circles differ ently coloured. NVIlen, after looking long at a coloured object, we direct the eye to another, WC attribute to it the colour of the first. In the Third book, which treats of mirrors, he shows, that in a plain mirror, the.object is seen in the perpendicular drawn from the object to the plane of the mirror, and continued behind it. He mentions, that objects appear smaller towards the zenith, and larger towards the horizon, because, in the former case we see them in a position to which we are less ac customed. In concave mirrors the objects appear con cave, anti in convex ones they appear convex, and the image is seen at the point of intefsection of the reflected ray, and the line drawn from the object to the centre of the sphere. The Fourth book treats of concave mirrors. In these mirrors an object may be reflected and render ed visible by all the parts of the mirror, or hy three, or two, or even one point. The image may be either on the surface of the mirror, or before the surface, or be hind the eye, or behind the mirror. When the image is beind the mirror, the distance of the object from the mirror is less than that of the image. When the image is between the eye and the mirror, the distance of the object from the eye will bc sometimes greater than the distance of the image from the mirror, and sometimes it will bc equal to it, and sometimes less. When the object is hetween the mirror and the eye, it will be seen in a part different from that where it really is ; and if we give it a motion in one direction, it will appear to MONT in the opposite direction. l'he Fifth book is the most curious and valuable of the whole work. Ptolemy begins by explaining the experiment with the piece of money, which, when concealed behind the sides of a ves sel, becomes visible by filling it with water. The re fraction of the visual ray in penetrating the water, makes us see the piece of money out of its true place, and in the prolongation of the primitive direction of the ray emitted from the eye. In order to measure this refraction at different angles, Ptolemy employs a circle divided into 360°, the inferior half of which is plunged in the water, so that the refracting surface covers one of the diame ters of the circle. The centre of the circle is marked by a small coloured body, and a second similar body is fitted to one of the quadrants out of the water, and at a given distance from the vertical diameter ; a third co loured body slides on the lower part, which is immersed in the water. This last body is then pushed with a rod till the eye placed on the body in the air sees all the thrce in a straight line. The two distances of the se cond and third body from the vertical diameter are then measured on the graduated circle.