REPORT ON PHYSrCAL OPTICS. 301 



produces the sensation of liglit. This supposition seents to be 

 supported by the discoveries of Herschel, Wollaston, and Ritter, 

 respecting tlie invisible rays of the spectrum ; but it does not 

 appear to be easily reconciled with any h3rpothesis which we are 

 able to frame respecting the nature of vision. This uniformity 

 of velocity, on the other hand, is a necessary consequence of 

 the principles of the wave-theory. Tlie velocity with which 

 vibratory movement is propagated in an elastic medium depends 

 solely on the elasticity of that medium and on its density ; and 

 if these be uniform in the vast spaces which intervene between 

 the material bodies of the universe, (and it is not easy to sup- 

 pose it otherwise,) the velocity must be the same, whatever be 

 the originating source. 



The rectilinear motion of light has long been ui'ged in favour 

 of the theory of emission, and against the theory of waves. If 

 light consists in the undulations of an elastic fluid, (it has been 

 said,) it should be propagated in all directions fi'om every new 

 centre, and so bend round interposed obstacles. Thus luminous 

 objects should be visible, even when an opake 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 ob- 

 jection, which was first insisted on by Newton*, a full answer 

 has been given. The phenomena of diff'raction, and especially 

 the interior fringes in the shadow of narrow opake bodies, prove 

 that light does bend round obstacles, and deviate perceptibly 

 from the rectilinear course. When the obstacle is of consider- 

 able 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 percep- 

 tible. 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 sura 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 inclined at a small angle to the normal to the front 

 of the wave. The accurate calculation of the intensity, in this 



* Optics, Query 28. 



