PRISMATIC REFRACTION. 121 



rays, which do not appear to be explained by the hypo- 

 thesis of emission or of undulation. 



In both theories a wave motion is admitted, and every 

 fact renders it probable that this mode of progression 

 applies not only to light, but to the so-called imponder- 

 able forces in general. Admitting, therefore, the undu- 

 latory movement of luminous rays, we shall not stop to 

 consider those points of the discussion which have been 

 so ably dealt with by Young, Laplace, Fresnel, Biot, 

 Frauenhofer, Herschel, Brewster, and others, but pro- 

 ceed at once to consider the sources of light, and its 

 more remarkable phenomena.* 



The sun is the greatest permanently luminous body 

 we are acquainted with, and that orb is continually pour- 

 ing off light from its surface in all directions at the rate, 

 through the resisting medium of space and of our own 

 atmosphere, of 192,000 miles in a second of time. It has 

 been calculated, however, that light would move through 

 a vacuum with the speed of 192,500 miles in the same 

 period. \Ve, therefore, learn that a ray of light requires 

 eight minutes and thirteen seconds to come from the sun 

 to us. In travelling from the distant planet Uranus, 

 nearly three hours are exhausted ; and from the nearest 



* On the two theories the following maybe consulted : Young, 

 Supplement to Encyclopedia Britannica, article Chromatics; Fres- 

 nel, Supplement a la Traduction Fran$aise de la bieme edition du 

 Traite de Chimie de Thomson, par Riffault, Paris, 1822 ; Herschel's 

 Article, Light, in the Encyclopaedia Metropolitan*, and the French 

 Translation of it by Quetelet and Verhulst; Airy's Tract on the 

 Undulatory Theory, in his Tracts, 2nd edition, Cambridge, 1831 : 

 Powel, The Undulatory Theory applied to Dispersion, &c. p. 184 ; 

 Lloyd's Lectures, Dublin, 1836-41 ; Cauchy, Sur le Mouvement des 

 Corps elastiques, Memoires de 1'Institut, 1827, vol. ix. p. 114; 

 Theorie de la Lumiere, Ibid. vol. x. p. 293; M'Cullagh, On Double 

 Refraction, Ibid., vol. xvi. ; Geometrical Propositions applied to the 

 Wave Theory of Light, Ibid., vol. xvii. ; Sir David Brewster's 

 papers in the Transactions of the Royal Society of Edinburgh, and 

 the Philosophical Magazine. 



