Astronomical and Nautical Collections. 451 



This ingenious explanation, suggested by Dr. Young him- 

 gelf to the advocates of the system of emission, presents 

 great difficulties when it is followed in its consequences, 

 and is compared with facts. But this is not the place for 

 examining them in detail ; the question is more particularly 

 investigated in a Memoir on Diffraction, which is to be 

 printed in the Collection des Savans Strangers. Besides, 

 the necessity of the discussion seems to be superseded by the 

 phenomena of diffraction which are about to be considered, 

 and which appear to be manifestly contradictory to the 

 system of emission. 



Dr. YouNO had supposed, and I had afterwards been of 

 the same opinion, without having been informed of what he 

 had published on the subject, that the external fringes are 

 produced by the meeting of the direct light with the rays 

 reflected on the termination of the opaque body; but, if this 

 had been the case, the edge of a razor, which presents so 

 very small a reflecting surface, should exhibit these fringes 

 much less conspicuously than the back of the razor, which 

 reflects so much more light ; while in fact we observe very 

 little difference in the intensity of the fringes thus produced, 

 unless we examine them very near to the razor. 



When we transmit the rays proceeding from a luminous 

 point, through an aperture about one-fiftieth of an inch in 

 breadth, and of any convenient length ; if the luminous 

 point is not too near to the aperture, we find, as we observe 

 at a greater and greater distance from it, the pencil of rays 

 spread very considerably, and illuminate a space on the 

 card, or form a stripe of light in the focus of the lens, 

 which is much wider than the conical projection of the 

 aperture, which would be formed by right lines drawn from 

 the point in the direction of tangents to the edges of the 

 aperture. 



We may consider the edges as very thin, like those of 

 two razors perfectly sharpened, in order to make our reason- 

 ing the simpler. Now, if only the rays, which actually 

 pass close to the edges, were subject to any kind of in- 

 flection, it is obvious that a very small part of the light 

 transmitted would be spread towards the shadow ; and the 

 inflected rays would exhibit but a very feeble light, in the 



