CONTKIBUTIONS TO SCIENCE. 



493 



superposition of the two conditions of strain due to the two 

 twists independently. The isochromatic curves thus ob- 

 tained are represented in Tig. 4, which is taken from that 



Fig. 4. 



in the Edinburgh Transactions. The points B 1? B 2 , corre- 

 spond to no retardation. These curves completely agree 

 with those obtained when the plate of isinglass is examined 

 with circularly polarised light. The advantage of employing 

 circularly polarised light lies in the fact that whatever may 

 be the direction of the lines of stress in the isinglass the 

 two component rays into which the light may be supposed 

 separated on entering the medium are of equal intensity, and 

 the colour therefore depends only on the state of strain and 

 not on the position of the plane of primitive polarisation. 



The last example of stress in an elastic solid is that of 

 a triangle of unannealed glass, in which " the lines of equal 

 intensity of the action on light are seen without interruption 

 by using circularly polarised light." They are represented in 

 Plate III. In Figs. 5 and 6 A, BBB, DDD, are the neutral 

 points, or points of no action on light, and CCC, EEE, are 



