272 THOMAS YOUNG. 



reflected from the two surfaces coalesced to enhance the 

 intensity. 



Young next applied the wave theory to account for 

 the diffraction or inflection of light : that is to say, the 

 effects produced by its bending round the edges of 

 bodies'. When a cone of rays, issuing from a very 

 minute point, impinges on an opaque body, so as to 

 embrace it wholly, the shadow of the body, if received 

 upon a screen, exhibits fringes of colour. They follow 

 so closely the contour of the opaque body, that Sir John 

 Herschel compared them to the lines along the sea- 

 coast in a map. If a very thin slip of card, or a hair, 

 be placed within such a cone, it is noticed that besides 

 the fringes outside the shadow, bands of colour occur 

 within it; the central, or brightest, band being always 

 white when white light is employed. It is a singular 

 and somewhat startling fact, that by the interposition 

 of an opaque body, say a small circle of tinfoil, the 

 point on which we should expect the centre of the 

 shadow to fall is, by the joint action of diffraction and 

 interference, illuminated to precisely the same extent 

 as it is when the opaque circle is withdrawn.* In refer- 

 ence to the interior fringes Young made the observa- 

 tion, which is of primary importance, that if you 

 intercept the light passing by one of the edges of tho 

 strip of card or of the hair, the fringes disappear. It 

 requires the inflection of the waves round both edges of 

 the object, and their consequent interference, to pro- 

 duce these fringes. 



Young's attempt to explain the phenomena of dif- 

 fraction was a distinct advance on the extremely arti- 

 ficial hypothesis of Newton. Still his attempt was not 

 so successful as his explanation of the colours of striated 



* A similar diffraction has been proved by Lord Raylcigh to 

 occur in the case of sound. 



