426 Prof. Stokes on the Vibrations of Polarized Light. 



Assuming Archdeacon Pratt's area 3977 miles (wliich, how- 

 ever, is irreconcileabic with his centre of gravity, deduced with- 

 out either rectangle or parabola), I find the pressure on the 

 crown of the arch to be G213 miles of rock. 



This cannot crush an igneous rock, which will bear 115,75.5 

 miles of its own material, as may be inferred from the following 

 experiment of Tredgold : — 



" It required a weight of 24,556 lbs. avoirdupois to crush a 

 cube of Aberdeen granite \~ inch side, and spec. grav. 2"625." 



In the equation (Phil. Mag. p. 347, foot of page) the coeffi- 

 cient 2 is left out before (/i-|-r/ versiu ^), an error which affects 

 nearly all the figures of page 348 ; but as this numerical inaccu- 

 racy does not seriously affect the result obtained by Archdeacon 

 Pratt, I lay no stress upon it. 



Tvinity College, Dublin, SamUEL Haughton. 



November 10, 1859. 



LXIV. On the hearing of the Phcenomena of Diffraction on the 

 Direction of the Vibrations of Polarized Light, with Remarks 

 on the Paper of Professor F. Eisenlohr. By Professor G. G. 

 Stokes*. 



THE appearance in the Philosophical Magazine for Septem- 

 ber of a translation of Professor F. Eisenlohr's paper in 

 the 104th volume of Poggendorff's Annalen, induces me to offer 

 some remarks on the subject there treated of. 



Had my paper " On the Dynamical Theory of Diffraction f " 

 been accessible to M. Eisenlohr at the time when he wrote, he 

 would have seen that I did not content myself with merely 

 resolving the vibrations of the incident light in directions parallel 

 and perpendicular to the diffracted raj^, and neglecting the former 

 component, as competent to produce only normal vibrations, but 

 that I gave a rigorous dynamical solution of the problem, in 

 which the normal vibrations, or their imaginary representatives, 

 as well as the transversal vibrations, were fully taken into 

 account, though the result of the investigation showed that, in 

 case of diffraction in one and the same medium (the only case 

 investigated), the state of j)olarizatiou of the diffracted ray was 

 independent of the normal vibrations. M. Eisenlohr's result, 

 on the other hand, confessedly rests on the assumption that the 

 diffracted ray may be regarded as produced by an incident ray 

 agreeing in direction of fropugation with an incident ray which 

 uould produce the diffracted ray by regular refraction, but in 

 direction of vibration (in the immediate neighbourhood of the 



* Communicated by the Author. 



t Cambridge Philosophical Transactions, vol. ix. p. 1. 



