1891.] 



On the Plasticity of an Ice Crystal. 



327 



of the middle crystal are in the same position as before the bending. 

 The question immediately suggested itself whether the bend was due 

 to a limited number of layers sliding over each other by finite amounts, 

 or to a true shearing strain. I examined the surfaces of the bubbles 

 very carefully with a magnifying glass, and could find no trace of pro- 

 jecting edges or " faults," so I concluded it was a true shear. My 



)lariscope was the same as was used two years ago. 



Light from the white paper A, fig. 2, was reflected by the three 



FIG. 2. 



lates of glass, B, upwards through the Nicol C, and then the ice was 

 lid on the glass stage E, or held in the closed hand. D was lined 

 nth black velvet. This simple apparatus served its purpose excel- 



itly, and it was seldom that I wished for a more elaborate apparatus 

 rith convergent light. The bent bar under this polariscope was 

 rand to have the optic axis as nearly as I could tsll normal to the 



VOL. XLIX. z 



