ON THE STRESSES IN SOLID BODIES DUE TO UNEQUAL HEATING, AND 

 ON THE DOUBLE REFRACTION RESULTING THERl-^FROM. 



LORD RAYLEIGH. 



The plienoinena of light and colour exhibited in the polariscope when 

 strained glass is interposed betweeu crossed niçois are \vell known to 

 every student of optics. The straiu raay he of a permanent character, 

 as in glass imperfectly annealed or specially unannealed^ or it may be 

 temporary, due to variations température or to mechanical force applied 

 from without. One of the best examples under the last head is that of 

 a rectangular bar subjected to tlexure^ the plane of the flexure being 

 perpendicular to the course of the light. The full eli'ect is obtained 

 when the length of the bar is at 45° to the direction of polarization. 

 The reviyal of light is a maximum at the edges^ where the material 

 traversed is most stretched or comjîressed, while down the middle a 

 dark bar is seen representing the „neutral axis." It is especially to be 

 noted that the etl'ect is due to the glass being nnequalli/ stretched in the 

 two directions perpendicular to the line of vision. Thus in the case 

 under discussion no force is o])erative perpendicular to the length of 

 the bar. Under a purely hydrostatic pressure the singly refracting cha- 

 racter of the material would not be disturbed. 



When a pièce of glass, previously in a state of ease, is nnequally 

 lieated, double refraction usually ensues. Tins is due, not directly to 

 the beat, but to the stresses, différent in ditlerent directions and at dif- 

 férent places, caused by the unecpial expansions of the varions parts. 

 The investigation of thèse stresses is a problem in Elasticity llrst attac- 



