Astronomical and Nautical Collections. 131 



book of his Optics, does not appear to have remarked the 

 internal fringes, although his researches were subsequent to 

 those of Grimaldi ; for he says in the 28th query of his third 

 book, where he states as an objection to the theory of undu- 

 lation, that the luminous waves ought to spread into the 

 shadow : " the rays which pass very near to the edges of 

 any body are bent a little by the action of the body, as we 

 showed above ; but this bending is not towards but from the 

 shadow, and is performed only in the passage of the ray by 

 the body, and at a very small distance from it : so soon as 

 the ray is past the body, it goes right on.'* It is difficult to 

 understand how the inflection of light into the shadow 

 should have escaped so accurate an observer, especially 

 when we consider that his experiments were made on very 

 narrow bodies, and sometimes even on hairs. We are almost 

 compelled to suspect, that his theoretical prejudices had, in 

 a certain degree, tended to shut his eyes to these important 

 phenomena, which contribute very much to do away the 

 objection, on which he so much depended for the superiority 

 of his system. 



Since this inflection of the light into the shadow is a lead- 

 ing fact in the theory, it becomes necessary to insist on the 

 details of the experiment by which it is established. In 

 order to make it in the most unexceptionable manner, let 

 the sun's light be admitted into a dark room through a hole 

 in the window-shutter, covered with a piece of tin foil, in 

 which a hole is made with a pin, not exceeding ^io of an inch 

 in diameter ; instead of allowing the sun to shine directly 

 on this hole, let the light be first reflected by a small mirror 

 into a direction nearly horizontal. Place now in the lumi- 

 nous cone, formed by the rays thus introduced, a perfectly 

 opaque wire, about ^^ of an inch in diameter : suppose the 

 distance of the iron from the hole to be about a metre or 

 about a yard, and let the shadow be received on a screen of 

 white paper at the distance of two metres or of two yards. 

 If the small hole were of evanescent dimensions, it is plain 

 that the geometrical shadow as projected on the card would 

 be ^3, of an inch in diameter, that is, supposing the rays to 

 sufler no inflection. 



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