ON TIIK STI'iKSSKS IX SOI.III l'.ODIKS l)l'K ÏO t'XKOl 'Al. IIKATIXG, AXI) OX TIIK 1IOITU.K IIEIT.AOTION l'.KSl I. I IXC Til K.liKI'ISOM.
The phenoinena of liglit mul colour evhibited in the polariseope wlien strained ulass is interposed bet ween crossed nicols are well knowu to every student ot' opties. The strain niuy he of' a permanent charaeter, as in glass iinperfectly annealed or specially unaimealed, or il may he temporarv, due to vurintions temperature or to mechanieal torce applied from without. One of the best examples under the last liead is tliut of a rectangular bar subjected to tlexure, the plane of the flexure being perpeiidieular to the course of the liglit- The ful'l eli'ect is obtained when the lengtli of the bar is at h>° to the direction of polarizatinn. The revival of liglit is u maximum at the edges, where the material traversed is most stretched or compressed, wliile down the middle a dark bar is seen representing the „neutral axis. It is espeeially to be noted tliat the eli'ect is due to the glass being nii*iually stretched in the two direetions perpeiidieular to the lint' ol vision. I bus in the case under discussion 110 force is operative perpeiidieular to the length ol the bar. l uder a purely hydrostatic pressure the singly refracting eliaracter of the material would not be disturbed.
\\ hen a piece of glass, previously in a state of case, is unequalh lieated, doublé refraction usually ensues. I liis is due, not directly to the heat, hut to the stresses, dill'erent in dillerent direetions and at different places, caused by the une(|ual expansions ot the various paris. The investigation of tliese stresses is a problein in hlasticity lirst attac-