curacy. See REFRACTION.
Contemporary with Vitellio was Roger Bacon, a man of very extensive genius, who wrote upon almost every branch of science; though it is thought his improve ments in optics were not carried tar be yond those of Alhazen and Vitellio : to him, however, has been attributed the invention of the MAGIC LANTERN, which see.
One of the next who distinguished him self in this way, was 11Iaurolycus, teacher of mathematics at Messina. In a treatise, " De Lumine et Umbra," published in 1575, he demonstrates, that the crystal line humour of the eye is a lens that col lects the rays of light issuing from the objects, and throws them upon the retina, where the focus of each pencil is. From this principle he discovered the reason why some people are short-sighted, and others long-sighted ; also why the former are relieved by concave glasses, and the others by convex ones.
Contemporary with Maurolycus was John Baptista Porta, of Naples. He dis covered the camera obscura, which throws considerable light on the nature of vision. His house was the constant resort of all the ingenious persons at Naples, whom he formed into what he called An Acade my of Secrets, each member being oblig ed to contribute something that was not gen erally known, and might be useful. By this means he was furnished with materials for his " Magia Naturalis," which con tains his account of the camera obscura, the first edition of which was publish ed, as he informs us, when he was not quite fifteen years old. He also gave the first hint of the magic lantern, which Kir cher afterwards followed and improved. His experiments with the camera obscura convinced him, that vision is performed by the intromission of something into the eye, and not by visual rays proceeding from it, as had been formerly imagined ; and he was the first who fully satisfied himself and others upon this subject. Ile justly considered the eye as a camera ob scura, and the pupil the hole in the win dow-shutter ; but lie was mistaken in sup posing that the crystalline humour corres ponds to the wall which receives the ima ges; nor was it discovered till the year 1604, that this office is performed by the retina. He made a variety ofjust remarks concerning vision, and particularly ex plained several cases in which we imagine things to be without the eye, when the appearances are occasioned by some af fection of the eye itself, or by some motion within the eye. He remarked also, that, in certain circumstances, vision will be assisted by convex or concave glasses; and he seems even to have made some small advances towards the discovery of telescopes. Other treatises on optics, with various and gradual improvements, were afterwards successively publiShed by se veral authors, whose names, with the titles and brief accounts of their general works, would occupy a large space. We must, however, mention the excellent work on optics, by Dr. Smith, 2 vols. 4to. ; an
abridgment of which was made by Dr. Kipling, for the use of the students at the Universities, entitled, " Elementary Parts of Dr. Smith's Optics," &c. 1778; and an elaborate History of the Present State of Discoveries relating to Vision, Light, and Colours, by Dr. Priestley, 4to. 1772 ; a work highly instructive and entertaining to persons who have a taste for physics.
The laws of optics depending upon the properties of Light, the reader will do well, as introductory to this article, to re fer to what has been said in our fourth volume on that subject. There will be found much curious speculation, and a variety of interesting facts relating to the nature of light, its velocity, and the direc tion which it takes in moving through free space and through our atmosphere. We shall in this place give a few definitions necessary to the mere student.
By a ray of light, is meant the motion of a single particle ; and its motion is re presented by a straight line. Any parcel of rays proceeding from a point, is called a pencil of rays. By a medium, is meant any pellucid or transparent body, which suffers light to pass through it. Thus, water, air, and glass, are called media. Parallel rays, are such as move always at the same distance from each other. if rays continually recede from each other, as from C to c d (Plate I. Optics, fig. 1.) they are said to diverge. If they continually approach towards each other, as in moving from c d to C, they are said to converge. The point at which converging rays meet, is called the focus. The point towards which they tend, but which they are pre vented from coming to, by some obstacle, is called the imaginary focus. When rays, after passing through one medium, on en tering another medium of different density, are bent out of their former course, and Made to change their direction, they are said to be refracted : thus AC (fig. 2), is a ray which, when it enters the medium IIGK, instead of proceeding in the same direction CL, it is made to move in the direction CS. When they strike against a surface, and are sent back again from the surface, they are said to be reflected. The incident ray, as AC, is that which comes from any luminous body, and falls upon the reflecting surface, as HK, and CAI is the reflected ray. The angle of incidence, is that which is contained be tween the incident ray AC and a perpen dicular to the reflecting surface in the point of reflection, as the angle ACD. The angle of reflection, is that contained between the said perpendicular DC, and the reflected ray CM, viz. the angle DC M. The angle of' refraction, is that contained between the refracted ray CS, and the per pendicular CN, viz. the angle FCK. The angle of deviation, is that which is con tained between the line of direction of an incident ray AL, and the direction of the same ray CI', after it is refracted ; thus the angle is the angle of deviation.