Rays, in reference to the Undulatory Theory of Light. 361 



optics ; and the recent beautiful discoveries of Professor Airy, 

 Mr. Hamilton, and Mr. Lloyd afford the finest examples of its 

 influence in predicting new phaenomena. The power of a 

 theory, however, to explain and predict facts, is by no means 

 a test of its truth ; and in support of this observation we have 

 only to appeal to the Newtonian Theory of Fits, and to Biot's 

 beautiful and profound Theory of the Oscillation of Luminous 

 Molecules. Twenty theories, indeed, may all enjoy the merit 

 of accounting for a certain class of facts, provided they have 

 all contrived to interweave some common principle to which 

 these facts are actually related. 



On these grounds I have not yet ventured to kneel at the 

 new shrine, and 1 must even acknowledge myself subject to 

 that national weakness which urges me to venerate, and even 

 to support, the falling temple in which Newton once wor- 

 shiped. 



That the undulatory theory is defective as a j)hysical repre- 

 sentation of the phaenomena of light, has been admitted by 

 the more candid of its supporters ; and this defect, in so far 

 as it relates to the dispersive power of bodies, has been stated 

 by Sir John Herschel as a " most formidable objection *." 

 That there are other objections to it, as a physical theory, I 

 shall now proceed to show ; and I shall leave it to the candour 

 of the reader to determine whether they are more or less for- 

 midable than that which has been stated. 



According to the Undulatory Theory, light consists in the 

 undulations of an exceedingly rare and elastic medium called 

 iEther, which pervades all space, and which exists in the in- 

 terior of all refractive media, but with a diminished elasticity, 

 the aether being least elastic in the most refractive substances. 

 As, in sound, the pitch or note is determined by the frequency 

 of the aerial pulses; so in light the colour is determined by 

 the frequency of the ethereal pulses. Generally speaking, in- 

 deed, light differs from sound, according to this theory, only 

 in the undulations being performed in media of very different 

 elasticities. 



If we transmit white light through the thinnest film, that can 

 be detached, of transparent native orpiment, the light will be a 

 bright greenishyellow; and if we analyse this light by the prism, 

 we shall find that it contains none of the violet rays. Hence 

 it follows, — and we find it so by direct experiment, — that this 

 thin transparent film is absolutely opake to violet light, refusing 

 to transmit a single ray of it through its substance. Now this 

 film contains aether which is freely put into undulation by red, 



* Treatise on Light, § 565. 

 TJiird Series. Vol. 2. No. 11. May 1833. 3 A 



