Royal Sucietij. 301 



rays passed by llie edge of the body, without suffering any deviation 

 from a rectilinear course. It is well known that, under these cir- 

 cumstances, the real shadow is broader than the geometrical 

 shadow, indicating a deflexion of the rays from the edge of the 

 intercepting body. By varying the distances at which the obser- 

 vations are taken, it is found that the rays are not bent at a sharp 

 angle, but pursue a curvilinear course, the concavity of which is 

 towards the shadow, the curve itself resembling an hyperbola. A 

 luminous halo also appears beyond the shadow; the breadth of 

 this halo agreeing accurately, at all distances, with the space which 

 the penumbra should occupy, if the rays were not bent. The author 

 thinks it impossible to reconcile the explanation of these phseno- 

 mena given by Newton, with his own hypothesis concerning the 

 action of solid bodies on light, as stated in the " Principia :" for, in 

 that hypothesis, the rays passing nearest to the edge of an intercept- 

 ing body are supposed to be bent towards the edge, as if attracted ; 

 whereas the explanation proceeds upon the supposition that they 

 are bent from that body, as if repelled. The actual hyperbolic 

 course of the rays is also inconsistent with that hypothesis, which 

 would assign to them a parabolic path. It also appears that the 

 breadth of the spectrum made by receiving the sun's rays through 

 an aperture one tenth of an inch, or more, in width, is less than if 

 the rays proceeded in straight lines ; but if the aperture is very 

 much dimini.shed, the result is reversed, the real spectrum being 

 broader than the geometrical spectrum. 



The author conceives, that the whole of the observed phaenomena 

 will admit of explanation, by assuming that light consists of mate- 

 rial particles, endowed with a power of mutual repulsion, in which 

 case they would obey the laws of elastic fluids ; and the course of the 

 rays might admit of comparison with the motions of the particles 

 of air, or other similarly constituted fluids, in flowing past an obstacle 

 opposed to their progress. He shows how this hypothesis furnishes 

 an explanation of the deflexion of the rays, and of the curvature of 

 their path ; and why that path resembles an hyperbola. He sup- 

 ports this theory by the analogy of the laws of heat, considered as 

 the properties of a material fluid, with those of light; both exhibit- 

 ing the pluenomena of reflexion, refraction, and polarization. The 

 author is inclined to believe that, besides the deflecting force, the 

 presence of which is already established, there exists also an in- 

 flecting force, which bends some of the rays towards the intercepting 

 body ; and states a variety of considerations in support of this fact. 

 Me explains, on the same principles, the phaenomena described by 

 Newton under the a|)pellation oi'Jits of easy reflexion and easy trans- 

 mission, which Dr. Young has explained on the undulatory theory, 

 by the principle of interferences; but which may be considered as 

 analogous to the alternating movements of elastic fluids striking 

 against an opposing body, or entering by a narrow aperture; move- 

 ments which, in air, give rise to vibrations constituting musical 

 Boimds. 



The Society then adjourned over the Long Vacation, to meet 

 again on the 17ih of November. Zooi.o- 



