858 ACROPHYSICS. 



trated experimentally the cause of the rainbow ; the complete theory 

 of which was afterwards given by Descartes. The law of refraction 

 was discovered by Snell, of Ley den, in 1621 ; and Bartholin, of 

 Denmark, first noticed the phenomenon of double refraction about 

 1669 : to which Huyghens added, that the light thus refracted, was 

 polarized at the same time. Grimaldi first noticed the diffraction of 

 light, in 1665 : and Newton, in 1675, studied the formation of colored 

 rings, and fringes, by means of thin plates. In 1672, Newton 

 announced to the Royal Society his new theory of light ; and its 

 application to the prismatic spectrum. The invention of achromatic 

 telescopes, belongs to Mr. Hall, of England, as early as 1733 ; 

 though first patented and made public by Mr. Dollond, in 1757. In 

 1800-8 Dr. Young applied the undulatory theory to the general 

 explanation of colors; and in 1810, Malus, of France, discovered 

 the polarization of light by reflection. Dr. Wollaston invented the 

 camera lucida, in 1807. The more recent discoveries made by Biot, 

 Brewster, and others, we have no room to describe. 



We proceed to give some ideas of this science, under the heads 

 of Catoptrics, Dioptrics, Physical Optics, and Practical Optics ; as 

 the more common division of the subject. 



1. Catoptrics, so named from xatorttpov, a mirror, treats of the 

 reflection of light from smooth or polished surfaces. This reflection 

 takes place according to the same law as in the case of sound, or of 

 elastic bodies. The incident ray, and the reflected ray resulting 

 from it, both lie in the same plane, perpendicular to the reflecting 

 surface ; and the angles which they form respectively with a line 

 perpendicular to the surface, that is, the angles of incidence and of 

 reflection, are always equal. A polished metallic reflector, is called 

 a speculum; and a glass reflector is usually called a mirror : but, in 

 the latter, the light is chiefly reflected from the silvering, or metallic 

 surface on the back, to which the glass gives shape and smoothness. 

 Reflectors are generally either plane, or concave, or convex ; the two 

 latter with spherical surfaces, and taking their name from the side 

 which reflects the light. In plane reflectors, the rays make the same 

 angle with each other, after, as before their reflection. 



From a concave reflector, parallel rays are reflected, converging, to 

 a point called the principal focus ; at which the heat and light are 

 concentrated. Rays which previously converged, are reflected by a 

 concave surface still more converging : but rays which diverged, before 

 impinging, will be reflected either less diverging, or parallel, or con- 

 verging, according to the distance of the radiant point from which 

 they emanated. If the luminous object be more distant from the 

 concave surface than the centre of curvature of the latter, then an 

 inverted and reduced image of the object will be formed, between 

 the centre of curvature and the principal focus : but if the radiant 

 object be brought between these two points, it will produce an 

 inverted and magnified image, farther off than the centre of curva- 

 ture. This latter, is the precise arrangement of the reflecting micro- 

 scope; and the former case, where the luminous object is very distant, 

 is the principle of the reflecting telescope. As the concave reflector 

 always renders the rays more converging, or less diverging, than pre- 



