451& Astronomical arid Nautical Collections. 



middle of which we should see, distinctly marked, the bright 

 stripe illuminated by the direct rays: while, in fact, at a 

 proper distance, we observe an illumination nearly uniform, 

 extending over a space much wider than the real projection 

 of the aperture. The dimensions of the aperture are here 

 assumed to be small, in order that the experiment may easily 

 be made in a dark room of fifteen or twenty feet in length ; 

 but when the luminous point is at an infinite distance, like a 

 star, we may obtain the same efi'ect with an aperture of any 

 greater magnitude, provided that we observe at a distance 

 sufficiently great. 



It follows from these experiments, that the rays of light 

 may be made to deviate from their primitive direction, by 

 the neighbourhood of a screen, not only when they come 

 into contact with the edges of the screen, but even at very 

 sensible distances from these edges. 



Let us now consider the consequences of this principle 

 upon the system of emission. If the particles of light are 

 disturbed in the direction of their motions by the influence 

 of the body, in passing at sensible distances from their 

 surface, we must necessarily suppose that this effect is pro- 

 duced by attractive or repulsive forces emanating from the 

 body, and extending to the distances in question, or we must 

 attribute it to little atmospheres, of a similar extent, pos- 

 sessed of different refractive powers from the surrounding 

 medium. But on either of these suppositions it would 

 follow, that the inflection of the rays should vary with the 

 form, the thickness, or the nature of the margin of the 

 aperture, in the experiment which has been last described ; 

 while, in fact, the most accurate measurements exhibit no 

 differences whatever arising from these circumstances, and 

 show that the dilatation of the luminous pencils depends 

 merely on the breadth of the aperture. Hence it may be 

 inferred, that the phenomena of diffraction are incapable of 

 being explained by the system of emission. 



As this objection appears to be of high importance, and 

 decisive of the question, I think it necessary to describe 

 more particularly some of the experiments which establish 

 the principle on which it depends. 



I caused a pencil of light to pass between two plates of 



