26 REPORT ON PHYSICAL OPTICS. 



and so bend round interposed obstacles. Thus, luminous objects 

 should be visible, "*even when an opaque body is between them and 

 the eye, just as sounding bodies are heard, though a dense body 

 intervene between them and the ear. To this objection, which 

 was first insisted on by Newton, * a full answer has been given. 

 The phenomena of diffraction, and especially the interior fringes 

 in the shadow of narrow opaque bodies, prove that light does bend 

 round obstacles, and deviate perceptibly from the rectilinear course. 

 When the obstacle is of considerable dimensions, the intensity of 

 the light decreases, indeed, very rapidly within the edge of the 

 geometric shadow ; so that at a very small distance from that 

 edge it is no longer perceptible. But the darkness does not arise 

 from the absence of luminiferous waves, but from the mutual 

 destruction of those sent there. In fact, if the surface of the wave 

 when it reaches the obstacle be divided into any number of small 

 portions, the motion of the ether at any point behind it is, by the 

 principle of Huygens, the sum of all the motions produced there 

 by these several portions, considered as separate centres of dis- 

 turbance ; and it is easy to show that, when the distance of the 

 point in question from the obstacle is a large multiple of the 

 length of a wave, the magnitude of this resultant must diminish 

 rapidly within the shadow, and the light become insensible when 

 the line drawn from that point to the edge of the screen is in- 

 clined at a small angle to the normal to the front of the wave. 

 The accurate calculation of the intensity, in this and other similar 

 cases, has been made by Fresnel by the aid of the principle of 

 interference, and the result is found to agree in the most complete 

 manner with observation, f 



The same principles apply to the aerial waves which constitute 

 sound ; and these too should present analogous phenomena. But 

 the scale is widely different. The length of an aerial wave is more 

 than 10,000 times greater than that of an ethereal undulation ; 

 and the distance of the ear from the obstacle must be augmented 

 in the same proportion, in order that the same conclusions may be 

 applicable to the two cases. 



According to this account, then, the right-lined propagation of 

 the rays of light is a consequence of the principle of interference, 

 combined with the principle of Huygens. A very different view 



* Optics, Query 28. 



t " Memoire sur la Diffraction," Memoires de V Ltxtitut, torn. v. 



