310 FOURTH REPORT 1834. 



destroy one another by interference, when the sine of the angle 

 of incidence is greater than the ratio of the velocities of propa- 

 gation in the two media, or the angle itself greater than the li- 

 miting angle of total reflexion *. It is here supposed that 

 the distance from the refracting surface is a large multiple 

 of the length of a wave. The conclusion does not apply 

 to points very near that surface ; and for such points, there 

 is reason to think, the law of refraction is more complicated. 

 Experience shows, in fact, that light may issue from the 

 denser medium, to an appreciable distance, when the incidence 

 exceeds the limiting angle of total reflexion. If two prisms, 

 whose bases are slightly convex, be put together, and the 

 inclination of these bases gradually changed while we look 

 through them, it will be observed that, beyond the limiting 

 angle, the light will still be transmitted in the neighbourhood 

 of the parts in contact. By measuring the breadth of this space, 

 and comparing it to the diameters of the coloured rings, Fresnel 

 found that the interval of the glasses, through which this devia- 

 tion from the ordinary law of refraction occurred, exceeded the 

 length of a wavef . The analysis of M. Poisson points also to 

 the same result, and it is proved that the second medium will 

 be agitated in the part immediately in contact with the first, this 

 agitation decreasing rapidly and becoming insensible at a very 

 minute distance from the surface. 



The laws of reflexion and refraction, then, follow from the 

 theory of waves, whether we suppose the vibrating medium, in 

 dense bodies, to be the body itself, the ether within it, or both 

 conjointly. Euler maintained the first of these opinions, and 

 believed that light was propagated through the gross particles 

 alone, in the same manner as sound. But this hypothesis is 

 contradicted by the most obvious facts ; and according to it, as 

 Dr. Young has observed, the refraction of the rays of light in 

 our atmosphere should be a million times greater than it is. Of 

 the other two opinions, Yoimg seems to have held the latter, 

 and to have thought that the molecules of the body formed, to- 

 gether with those of the ether within it, a compound vibrating 

 medium, which was denser than the ether alone, but not more 

 elastic. Others, lastly, attribute the propagation of light in 

 transparent bodies to the vibrations of the ether alone, that fluid 

 being retained bj^ the attraction of the body in a state of greater 

 density within it than in free space. 



A very diiferent view of this subject has been recently main- 



* Sec Fresnel " Stir le Systeme dcs Vibrations lumineuses," Bibiwtluquc 

 Universelle, torn. xxii. 

 t JbiJ. 



