OPTICS. 359 



viously ; so the convex reflector renders them more divergent, or less 

 convergent, than they were before reflection ; and hence forms 

 reduced images of objects, apparently behind the mirror. 



2. Dioptrics, from 6<,o7trpa, a perspective instrument, relates to 

 the phenomena of light transmitted through transparent bodies. When 

 a ray of light impinges obliquely on a transparent body, that part 

 which passes through it, is refracted, or bent from its previous course, 

 both on entering and on leaving the body. Thus, if we look at any 

 object at the bottom of an empty vessel, and watch it while water is 

 poured into the vessel, the object will seem to rise. If the transpa- 

 rent medium be of uniform density, the ray, while in it, will move 

 in a straight line : but if the density vary, the ray will describe a 

 curve, as in the atmosphere, where the rays of light are bent down- 

 ward. The angle formed by the refracted ray with the perpendicular 

 to the surface, is called the angle of refraction: and, for the same 

 media, the sines of the angles of incidence and of refraction have a 

 constant ratio, whatever be the obliquity. This ratio, called the 

 index of refraction, is, at the common surface of air and water, 

 1.336 ; and for air and glass it is about 1.5 ; varying with the com- 

 position of the glass employed. 



These facts apply immediately to lenses ; which are, usually, cir- 

 cular pieces of glass, having one at least of the two opposite surfaces 

 spherical. The double convex, plano-convex, and meniscus lenses, 

 are thickest in the middle, and have a converging effect on the rays 

 which pass through them ; while the double concave, plano-concave, 

 and concavo-convex lenses are thinnest in the middle, and tend to 

 make the rays which pass through them divergent. The focus of 

 parallel rays passing through a converging lens, is called the prin- 

 cipal focus ; but as the radiant point, or object, draws nearer, the 

 focus is carried farther off, and, beyond a certain distance, the image 

 is magnified. This is the leading principle of the compound re- 

 fracting microscope. If an object be brought too near to the eye, 

 the rays from each point diverge too much for distinct vision ; but a 

 converging lens interposed enables the object to be seen distinctly 

 when much nearer, and thus makes it appear larger. This is the 

 principle of the simple refracting microscope, and of all convex 

 eye-glasses. In the refracting telescope, an image of the distant 

 object is formed by the rays passing through the object glass ; and 

 this image is seen magnified, through the eye-glass, placed very near 

 it. Objects seen through a concave, or diverging lens, appear nearer 

 and smaller than they really are. 



3. Physical Optics, is that branch of this science, which treats 

 of the physical properties of light, and the means by which they 

 have been investigated. Light may be decomposed, either by refrac- 

 tion, absorption, or reflection. If a ray of light be made to pass 

 through two faces of a triangular glass prism, it will not only be re- 

 fracted, or turned away from the vertex of the angle formed by 

 those two faces, but it will be decomposed into rays of seven distinct 

 colours ; red, orange, yellow, green, blue, indigo, and violet : and if 

 these colours be received on a screen, or card, they will form what 

 is called the prismatic spectrum. Red, yellow, and blue, are gene- 



