204 Obituary Notices of Fellows deceased. 



corresponding to the incident light ; let Oi be any point in the plane P, 

 (IS an element of that plane adjacent to Oij and consider the dis- 

 turbance due to that portion only of the incident disturbance which 

 passes continually across dS. Let O be any point in the medium 

 situated at a distance from the point G! which is large in comparison 

 with the length of a wave ; let OOi = r, and let this line make angles 

 with the direction of propagation of the incident light, or the axis 

 of x, and with the direction of vibration, or the axis of z. Then 

 the displacement at will take place in a direction perpendicular 

 to OOi, and lying in the plane ZOiO ; and if be the displacement 

 at Oi reckoned positive in the direction nearest to that in which the 

 incident vibrations are reckoned positive, 



(1 + cos 0) sin . /' (bt-r). 



4?rr 



In particular, if 



f(U - x) = c sin - 2 .5. (U - x), 



A, 



we shall have 



t - (1 + cos 0) sin . cos ?-~ (bt - r)." 



^j A/ "7* A- 



Stokes' own experiments on the polarisation of light diffracted by 

 a grating led him to the conclusion that the vibrations of light are 

 perpendicular to the plane of polarisation. 



The law of the secondary wave here deduced is doubtless a 

 possible one, but it seems questionable whether the problem is really 

 so definite as Stokes regarded it. A merely mathematical resolution 

 may be effected in an infinite number of ways ; and if the problem is 

 regarded as a physical one, it then becomes a question of the 

 character of the obstruction offered by an actual screen. 



As regards the application of the phenomena of diffraction to the 

 question of tne direction of vibration, Stokes' criterion finds a better- 

 subject in the case of diffraction by very small particles disturbing an 

 otherwise uniform medium, as when a fine precipitate of sulphur falls 

 from an aqueous solution. 



The work already referred to, as well as his general reputation, 

 naturally marked out Stokes for the Lucasiari Professorship, which fell 

 vacant at this time (1849). It is characterised throughout by accuracy 

 of thought and lucidity of statement. Analytical results are fully 

 interpreted, and are applied to questions of physical interest. Arith- 

 metic is never shirked. 



Among the papers which at this time flowed plentifully from his 

 pen, one " On Attractions, and on Clairaut's Theorem " deserves special 

 mention. In the writings of earlier authors the law of gravity at the 

 various points of the earth's surface had been deduced from more or 



