LIGHT. 27 



Sir Isaac Newton observed this fact with respect to the diamond, 

 which he thought was probably "an unctuous substance coagulated," 

 thus anticipating the discovery of its inflammability.* L. u. K. 



8. Light suffers reflection. 



The angles of incidence and reflection are always equal, as is 

 observed in a common plane mirror ; when two persons on opposite 

 sides, standing each at the same angle, see each others images. 



9. All objects seen by refraction or reflection appear in the direc- 

 tion of the refracted or reflected ray. 



This is confirmed by constant experience. 



10. Light undergoes polarization. f 



" This name has been given to a property of light, which causes 

 it often to be divided into two portions, one of which is transmitted, 

 the other reflected by the same pane of glass : or one portion sus- 

 tains refraction in an ordinary degree, the other in an extraordinary 

 degree. Again, all these properties are found to be commutable ; 

 so that the portion of the rays which is reflected in one case, may 

 be transmitted in another ; or that which in one case sustains the or- 

 dinary refraction, in another, may undergo the extraordinary refrac- 

 tion, and vice versa. 



These phenomena are ascribed to the different positions assumed 

 by different sets of rays ; certain poles, which they are supposed 

 to possess, being variously directed at different times, so as to de- 

 termine their reflection, or transmission, or the degree of their refrac- 

 tion.'^ This topic belongs to optics.^ 



11. Light produces little or no heat. 



The Lunar focus has always been said to exhibit no heat that can 

 be indicated by the most delicate thermometer ; and that whether 

 the rays were collected by a lens or mirror. No heat was felt in the 

 pupil of Sir Joseph Banks' eye, from the lunar rays collected by 

 Parker's great burning lens. 



But Dr. Howard, of Baltimore, by using his very delicate differ- 

 ential thermometer, filled with etherial vapor, || apparently found a 

 little heat in the moon's rays. 



The lunar light is composed of all the seven colors, as is evident 

 in the lunar bow, and in the lunar circles. IT 



* Dr. Brewster states that realgar, (red sulphuret of arsenic,) and chromate of lead, 

 exceed the diamond in refractive power, and all other substances in dispersive 

 power. PA. Tr. 1813. 



t For an account ot this curious property of light, the reader is referred to Henry's 

 Chemistry, 10th Edit. Vol. I. p. 154. Also Edin. Enc. Article Optics. Nich. Jour. 

 Vol. XXIII, p. 334, and 94th Vol. of the Annales de Chimie, Ure's Diet. 3d Edit. 

 568, and Cambridge Course of Mathematics. t Hare's Comp. 



All transparent crystals polarize light, except those whose primary form is the 

 cube or regular octohedron. Iceland crystal (rhomboidal calc-spar) is by far the 

 most energetic. 



}1 Am. Jour. Vol. II. p. 329. <T Am. Jour. Vol. XIV, p. 397. 



