578 



Professor Ernest G. Coker 



[Feb. 18, 



at right angles, and their optical effect is therefore proportional p - q. 

 The value of the stress difference may therefore be determined by 

 matching the optical effect produced at the given point with that 

 produced on a simple tension or compression member. The eve 

 is, however, not a very reliable instrument, especially after the fatigue 

 of a few minutes' exposure to a brilliant light ; and it is better still 

 to reduce the optical effect to zero by a simple tension or compres- 

 sion member set along one of the directions of principal stress, and 

 stressed until a dark field is produced. 



The laws which the optical effects obey may be at once utilized 

 for a variety of cases of practical interest. 



Thick Cylinder. 



An example is furnished by piping for transmitting fluids under 

 pressure. The action of water, or other fluid, in a pipe, can be 

 imitated by applying a uniformly distributed pressure to the interior 



Fig. 2. 



of a ring, such as is now shown, where the application of a uniformly 

 applied pressure produces a stress distribution in the circular ring of a 

 perfectly symmetrical character. The arrangement of the colour 

 bands shows that there is a very large stress at the interior surface, 

 diminishing rapidly at first, and afterwards more gradually as the 



