4:22 Mr. P. Cooper on the Connexion between 



posed any difficulty upon us, it is in explaining the appear- 

 ance of three orders of fringes, at certain relative distances 

 from each other. 



The light we have been treating of is so constituted, that 

 wherever the continuity of its surface is broken, the inter- 

 val produced by the interruption, as the light proceeds, 

 will be fringed with colours, which will increase in breadth 

 by increase of distance until the whole space is covered. 



Now, supposing we were at liberty to adopt the hypo- 

 thesis that bodies are surrounded by alternate spheres of 

 inflexion and deflexion, at different distances from their 

 surfaces ; and then assume that the spheres of inflexion 

 were situated at distances corresponding with the intervals 

 between the fringes, it would be a complete solution of the 

 the difficulty ; fringes would be formed upon the opposite 

 edges of the intervals, complementary to each other, which 

 before their complete developement, would be separated by 

 the white light observed by Grimaldi ; and the light in- 

 flected, carrying with it its divergent state, would form 

 the colours observed by him within the shadow. 



But there are several objections to this explanation, and 

 a very principal one is, that the existence of these spheres 

 of inflexion and deflexion has never been proved; and it 

 is evident, they are assumed for the purpose of giving a 

 plausible explanation of appearances which, it appears to 

 me, may be much better accounted for upon the known 

 principles of refraction. 



Before we proceed with this part of the subject, it is 

 desirable to inquire more particularly, by what force, or 

 by what means, light is diffracted bypassing through small 

 apertures. The principle upon which this light is formed, 

 when it is produced by passing through a lens, has been 

 sufficiently explained, and a little consideration will ren- 

 der it evident, that light which passes through a small 

 round aperture, is diffracted by the same force, differently 

 applied, but so as to produce a precisely similar arrange- 

 ment. 



It is generally admitted that the refraction of light com- 

 mences at some little distance from the new medium, and 

 continues through the same distance within it; increasing 

 from nothing to a maximum as it enters the medium, and 



