OPTICS. 



of mathematics at Messina. In a treatise, 

 " I)e 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. He 

 justly considered the eye as a camera ob- 

 scura, and the pupil the hole in the win- 

 dow-shutter ; but he 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 of just 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, witli 

 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 

 HGK, 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 UK, and 

 CM 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. 



