454 Astronomical and Nautical Collections, 



I shall mention another experiment, which fully demon- 

 strates that the mass and the nature of the edges of the 

 screen exert no appreciable influence on the deviation of the 

 rays of light. 



I covered a plate of glass with a coat of Indian ink, laid 

 on a thin sheet of paper, making together a thickness of 

 ^^ of an inch, and having cut two parallel lines through 

 them with the point of a penknife, I removed the strip of 

 blackened paper from the space between them. Having 

 measured the breadth of this space with the micrometer, 

 I formed a similar aperture by means of two cylinders of 

 solid brass, each about half an inch in diameter; and I 

 placed them by the side of the blackened glass, and at the 

 same distance from the luminous point. Observing, then, 

 by means of the micrometer, the pencils of light which 

 had passed through the two apertures, I found the dila- 

 tation of the breadth exactly the same in both cases. Now, 

 with respect to the mass of the substances forming the edges 

 of the two apertures, it would be difficult to find them 

 under circumstances more dissimilar: in one case the dif- 

 fraction was produced by the edges of a simple coat of 

 Indian ink united to a thin piece of paper, since the glass 

 could exert no action in either direction, being continued over 

 the whole aperture : in the other the light was inflected by 

 two cylinders of brass, which presented considerable masses 

 and considerable surfaces to the rays in their neighbourhood. 



It is therefore sufficiently proved, that neither the nature of 

 the bodies, nor the mass nor thickness of their terminations, 

 exhibits any sensible influence on the deviation of the rays 

 of light passing near them ; and it is equally obvious, that 

 this remarkable circumstance is incapable of being recon- 

 ciled with the system of emanation. 



The theory of undulation, on the contrary, afibrds us an 

 explanation of it, and even furnishes us with the means of 

 calculating all the phenomena of difi*raction ; so that the 

 results of the calculation are perfectly confirmed by the 

 observations ; as I have shown in the extract of the Memoir 

 on Difi"raction, published in the Xlth volume of the Annales 

 de Chimie et de Physique. 



[To be continued.] 



