author has been enabled to perform the experiment, or rather a mo- 

 dification of it, by the kindness of his friends Professors Miller and 

 O'Brien; of whom the former lent him a fine glass-grating, con- 

 sisting of a glass plate on which parallel and equidistant lines had 

 been ruled with a diamond at the rate of 1300 to the inch, and the 

 latter lent him the graduated instruments required. The theory 

 does not quite meet the case of a glass -grating, in which the diffrac- 

 tion takes place at the common surface of two media, but it leads to 

 a definite result on each of the two extreme suppositions : — 1st, that 

 the diffraction takes place before the light reaches the grooves ; 2nd, 

 that it takes place after the light has passed them ; and the results are 

 very different according as one or other of the two rival theories is 

 adopted. In the principal experiments, the plane of the plate was 

 placed perpendicular to the incident light, and the light observed 

 was that which had been diffracted by transmission through the 

 plate. The angle of diffraction, by which is meant the angle mea- 

 sured in air, ranged in the different experiments from about 20° to 

 60°. The result obtained was, that when the grooved face was 

 turned towards the eye, there was a very sensible crowding of the 

 planes of polarization of the diffracted light towards the plane of 

 diffraction. When the grooved face was turned towards the incident 

 light, there was a considerable crowding in the same direction, much 

 more than in the other case. Since the effect of refraction, con- 

 sidered apart from diffraction, would be to crowd the planes in the 

 contrary direction, the result seemed decisive in favour of Fresnel's 

 hypothesis, that the vibrations are perpendicular to the plane of po- 

 larization. On the other hypothesis, diffraction would have con- 

 spired with refraction to produce a large crowding in a direction 

 contrary to that in which the observed crowding took place. The 

 amount of crowding, in both positions of the plate, was nearly what 

 would be given by theory, on adopting Fresnel's hypothesis, and 

 supposing that the diffraction took place before the light reached the 

 grooves, but appeared in both cases a little less. The difference, how- 

 ever, was comprised within the limits of uncertainty depending upon 

 the errors of observation and the error in the assumed value of the 

 refractive index of the glass plate. 



December 10, 1849. 



Impact on Elastic Beams. By Homersham Cox, Esq., B.A., Jesus 

 College. 



Among the experiments instituted by the Royal Commission ap- 

 pointed to inquire respecting the use of iron in railway structure, 

 was a series relating to impact on beams. These experiments were 

 undertaken by Professor Hodgkinson, and were conducted in the 

 following manner. The two ends of the beam were fixed in a hori- 

 zontal position, and the blow was given against one of its vertical 

 sides in a horizontal direction. The instrument for giving the blow 



