Optical Determination of Stress* 749 



when these loci are determined with reference to the angular 

 positions of the nicols. Snch curves have been determined 

 graphically in this case and are shown by fig. 5. 



For measurement of the stress it is necessary to get rid 

 of the distorting effect produced by the black cross, and I am 

 greatly indebted to Professor Silvanus Thompson, F.R.S., 

 for suggesting for this purpose the use of two quarter wave- 

 plates (H, I, tig. 1) set with their axes inclined at 45° to the 

 Nicol's prisms, whereby the plane wave issuing from the first 

 nicol B is converted into circularly polarized light. The 

 circularly polarized beam produced by this combination, 

 whether right-handed or left-handed, has no special direction 

 of polarization, aud it therefore presents the same aspect to 

 all parts of the object under stress. It is again converted 

 to plane-polarized light by the inverse combination of quarter 

 wave-plate and nicol and the interference fringes are still 

 produced, while the black cross disappears. This arrange- 

 ment has the further practical advantage that, except for a 

 slight and invariable change of tint, the interference colours 

 produced are independent of the angular position of the 

 object. 



Fig. 6 shows the general arrangement of the colour fringes 

 presented by the hook when viewed by circularly polarized 

 light, and the stresses at points of the central section can be 

 determined in a similar manner to that described above. 



