6io 



ADAMS AND WILLIAMSON: BIREFRINGENCE AND STRESS 



as possible, two opposite faces being polished and the other 

 four faces left with a finely ground surface. Each block was in 

 turn subjected to compression in a Riehle vertical testing ma- 

 chine, and a beam of light polarized by a nicol prism was passed 

 through the block, the plane of polarization of the light being 

 at 45° to the direction of pressure. The blocks of glass when 

 loaded showed a certain amount of double refraction which 



Fig. I. 



Diagrammatic representation of apparatus for measuring birefringence of glass 

 under load. Pressure is transmitted to the block of glass G through the hardened 

 steel blocks A A and B B. Light from the lamp 5 is polarized by the nicol prism 

 N, passes through the glass, the lens L, the graduated quartz wedge W, the eyepiece 

 E and the analyzing nicol iVj. The nicols are crossed and the axis of thrust makes an 

 angle of 45 ° with the vibration direction of the light entering the block G. 



could be detected by the use of polarized light, and could be 

 measured by determining the optical path difference^ according 



- If two rays of light travel with velocities Va and Vb through the distance /, we 



Vb lla Na Xft 



have - = ~ = t; = — > >^a and tib being the corresponding refractive indices, 



Va Hb Nb Xo 



Xa and \b the wave-lengths, and Na and Nb the total numbers of waves. More- 



/ ftal 



over, Na = — = — m which Xi is the wave-length corresponding to w = i . Simi- 



Xa Xi 



flbl 



larly Nb = — and 

 Xi 



Na— Nb == 



(lla nb)l 



The product tial or 7ibl, respectively, is called the "optical path;" (wo — nb)l is 

 the "optical path-difference" and is the quantity which we measure. Obviously 

 the birefringence iia — iib may be obtained by dividing the optical path-difference 

 by the geometrical length of path. 



