smith: polarized skylight and the microscope 233 



loss of light, even when the nicols are crossed. The writer 

 has used such a compensator in his own work and finds it entirely 

 satisfactory. 



Fig. 2. Diagram to illustrate the action of the 

 polarized-skylight compensator or half-wave plate. 

 S-S is the trace of the plane of vibration of polarized 

 skylight. P-P is the trace of the plane of vibration 

 of the polarizer of the microscope. C-C and c-c are 

 the traces of the planes of vibration in the compensa- 

 tor. They bisect the angles between S-S and P-P. 

 The polarized skylight which enters the compensa- 

 tor, vibrating in the direction S-S and with ampli- 

 tude OS, is resolved into two plane polarized rays, 

 C-C and c-c, with amplitudes OC and Oc, respec- 

 tively, and with no phasal difference. Emerging 

 with a phasal difference of one-half wave length, 

 these rays combine to form a single plane polarized 

 ray, R-R, whose direction of vibration coincides with 

 that of the polarizer, P-P, and whose amplitude, OR, 

 equals that of the original ray, OS. Theoretically, 

 therefore, there is no loss in the light entering the 

 polarizer. 



The compensator just described can be used, of course, only 

 with a stationary polarizer. In order to use it with the type of 

 microscope in which both nicols can be rotated simultaneously, it 

 is necessary to connect the mounting of the compensator with that 

 of the polarizer in such a way that, when the polarizer is turned 

 through any angle, the compensator will be rotated, automatically, 

 through half that angle, the movements of both being in the same 

 direction. This rotation of the mounting, however, is inde- 

 pendent of that of the compensator itself, already referred to. 

 When both nicols are rotated simultaneously without the com- 

 pensator, the intensity of the light transmitted by the polarizer 

 may vary considerably, while with the compensator the illumi- 

 nation is uniform during rotation. 



It may happen, during the middle of the day, that the full 

 skylight, even from the zone of maximum polarization (which, it 

 may be noted, is also the zone of minimum illumination), is too 

 intense for the most satisfactory work with the petrographic 

 microscope. At such times, when using light from this zone or 



