108 The Lumini/erous Medium, 



The publication of Young's papers occasioned a fierce attack 

 on him in the Edinburgh Review, from the pen of Henry 

 Brougham, afterwards Lord Chancellor of England. Young 

 replied in a pamphlet, of which it is said* that only a single 

 copy was sold ; and there can be no doubt that Brougham for 

 the time being achieved his object of discrediting the wave- 

 theory, f 



Young now turned his attention to the fringes of shadows. 

 In the corpuscular explanation of these, it was supposed that 

 the attractive forces which operate in refraction extend their 

 influence to some distance from the surfaces of bodies, and 

 inflect such rays as pass close by. If this were the case, the 

 amount of inflexion should obviously depend on the strength of 

 the attractive forces, and consequently on the refractive indices 

 of the bodies a proposition which had been refuted by the 

 experiments of s'Gravesande. The cause of diffraction effects 

 was thus wholly unknown, until Young, in the Bakerian lecture 

 for 1803,J showed that the principle of interference is concerned 

 in their formation ; for when a hair is placed in the cone of rays 

 diverging from a luminous point, the internal fringes (i.e. those 

 within the geometrical shadow) disappear when the light passing 

 on one side of the hair is intercepted. His conjecture as to the 

 origin of the interfering rays was not so fortunate ; for he attri- 

 buted the fringes outside the geometrical shadow to interference 

 between the direct rays and rays reflected at the diffracting 

 edge ; and supposed the internal fringes of the shadow of a 

 narrow object to be due to the interference of rays inflected by 

 the two edges of the object. 



The success of so many developments of the wave-theory 

 led Young to inquire more closely into its capacity for solving 

 the chief outstanding problem of optics that of the behaviour 

 of light in crystals. The beautiful construction for the extra- 



* Peacock's Life of Young. 



t" Strange fellow," wrote Macaulay, when half a century afterwards he 

 found himself sitting beside Brougham in the House of Lords, " his powers 

 gone : his spite immortal." 



I Phil. Trans., 1804; Young's Works, i, p. 179. 



