1879.] Optical Study of the Elasticity of Solid Bodies. 193 



become, by iucrcasing forces, more and more regular, aud take the 

 form of conjugate hyperbolas, the axes of which being parallel aud 

 per])endicular to the main dimension of tlie rod. 



The trigonometrical tangent of the semi angle of the common 

 asymptote converges towards the value h ; therefore, the ratio of the 

 curvatures, aud consequently the ratio in question, is ^ with the best 

 isotropic body. 



The theoretical solution of the problem seems to bo iu favour 

 of Navier's aud Green's theories. 



Generality of the Optical Metliod. 



Ajiplication to the torsion of a rectangular plate. 



The shape of the deformed surface becomes a hyperbolic para- 

 boloid ; but the asymptotes of the hyperbolae (and not the axis, as 

 before) are parallel and perpendicular to the main dimension of the 

 rod. 



Fixing of the Newton's Rings by Photograpliy. 



In order to study at leisure, and with accuracy, the topographic 

 surfaces of elastic deformation, it is very convenient to keep an exact 

 and fixed image of the field. 



The induction spark between two poles of magnesium supplies a 

 source of light which fulfils the three necessary conditions — to be 

 intense, photographic, and monochromatic. 



Amongst the bright lines of the magnesium spectrum none is 

 useful for that purpose ; the radiation utilized as a source of photo- 

 graphic light is invisible, but becomes visible when projected on 

 Prof. Stokes's fluorescent screen. 



Though the photography of the Newton's rings be a delicate 

 o2)eratiou, the experiment will be tried before the audience. 



Newton's rings were photographed for the first time by the 

 illustrious Dr. Young at the Koyal Institution in the year 1803. 



