THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



OCTOBER 1888. 



XXXVI. Experiments on the Birefringent Action of Strained 

 Glass. By John Kerr, LL.D., Free Church Training 

 College, Glasgow*. 



1. rpHE action of strained glass in the polariscope was 

 J- discovered by Sir David Brewster, and studied by 

 him at some length, about seventy years ago. In the most of 

 his measurements and specifications of the effects of strain, 

 Brewster depended upon the observation of tints in the 

 polariscope, his optical means and methods being the same 

 for strained media as for doubly-refracting crystals. Plates 

 of glass were strained, either by ordinary forces or by local 

 changes of temperature, and were placed in the polariscope, 

 in combination with plates of sulphate of lime, or other 

 crystals, whose birefringent actions were known in kind and 

 quantity. In this way it was found possible to determine, 

 with a good degree of approximation, the difference of retar- 

 dations of two oppositely polarized rays, which passed through 

 the strained medium without being sensibly separated. 

 Brewster's principal results will come under our notice as 

 we proceed f. 



Fresnel contributed little to this part of Physical Optics 

 except his one experiment, in which, by the action of a train 

 of longitudinally compressed prisms, he succeeded in decom- 

 posing a pencil of common light into two separate pencils 



* Communicated by the Author. 



t The most of Sir Band's work on the subject is described in two of 

 his papers, in the Phil. Trans, for the year 1816. 



Fhil. Mag. S. 5. Vol. 26. No. 161. Oct. 1888. Y 



