Optical Determination of Stress. 



745 



convenient form of apparatus for this work is described 

 by Cheshire*, and his arrangement was used for the 

 experiments. 



I£ the value o£ the optical coefficient is known, and the 

 retardation is measured for a given thickness of the material, 

 the difference between the principal stresses at a point may 

 be obtained, or in the case of simple tension or compression 

 where one of the principal stresses is zero the absolute value 

 of the stress can be directly determined. 



For most purposes, however, it is more convenient to pro- 

 ceed in a different manner by a process of comparison of 

 colours. A scale may be readily formed in which the relation 

 between say tension or compression and colour due to inter- 

 ference is obtained experimentally. Thus, for example, a 

 specimen 0*49 in. wide and 0*123 in. thick gave the following 

 colour-scale in tension. 



Table III. 



Load in pounds 

 per sq. inch. 



Colour. 



Load in pounds ! 

 1 per sq. inch. 



Colour. 







Dark field. 



2020 



Reddish yellow. 



340 



Faint white. 



2170 



Reddish purple. 



670 



Intense white. 



2350 



Purple. 



1018 



Paint yellow. 



2510 



Sky-blue. 



1340 



Lemon-yellow. 



2690 



Very light blue. 



1680 



Orange-yellow. 



3020 



Nearly white. 



and these loadings were repeated at different times with the 

 same results as far as could be judged. 



It is difficult, however, to independently estimate most of 

 these colours, with the possible exception of purple, which is 

 usually well defined; and it is necessary for accuracy to 

 arrange the experiment, so that the colours produced in the 

 object under examination may be directly compared with a 

 colour produced by uniform stress. 



It is not difficult to arrange a standard specimen and the 

 object under test so that they may appear close together in 

 the same field of view, and this allows of much greater 

 accuracy as the colours produced may be matched in the 

 same manner as in photometric work. 



* " Some new Optical Projection Apparatus," by F. J. Cheshire. 

 Optical Society, 1908. 



