386 F. E. Wright — Measurement of Extinction Angles. 



results and to equal in sensitiveness any of the methods, whether 

 of local or of general application. 



2. The Adjustment of the Petrographic Microscope, with Special 

 Reference to the Measurement of Extinction Angles. 



A petrographic microscope in perfect adjustment should satisfy 

 the following requirements : (1) Its optical system should be 

 accurately centered ; (2) the axes of revolution of all revolvable 

 parts, whether stage or ocular, should coincide with the optic 

 axis of the microscope ; (3) the principal planes of the nicols 

 when crossed should be precisely 90° apart ; (4) the crosshairs 

 of the ocular should be parallel with the principal planes of 

 the nicols. Of the four conditions, the first two can be 

 accomplished without difficulty, and with the adjustment 

 screws fitted on every petrographic microscope. The last two, 

 however, require special appliances for accurate adjustment, 

 and without these can be effected only with difficulty. 



The test usually applied in ascertaining the correct position 

 for crossed nicols is that of the Bertrand ocular. A cap nicol 

 is used over the ocular and turned until the field of the Ber- 

 trand ocular shows uniform intensity of illumination through- 

 out. This can be accomplished readily and with an error of 

 less than ±15' if strong illumination be used. The cap nicol 

 is then revolved through an angle of 90°, the lower nicol 

 removed in its carriage, the upper nicol inserted and tested by 

 the cap nicol in its new position and adjusted until it is 

 actually crossed. The Bertrand ocular, however, furnishes 

 only one angle of rotation for the emerging waves, and allows 

 of no variability in this angle to meet different conditions in 

 the best way possible. This can be accomplished, however, 

 by use of the circularly polarizing bi-quartz wedge plate 

 described above or the bi-nicol ocular. With the bi-quartz wedge 

 plate the cap nicol is unnecessary and crossing of the micro- 

 scope nicols can be tested directly. For this purpose, parallel 

 light should be used and the entire lens system, both conclen- 

 sor lenses, objective and ocular, removed from the microscope ; 

 parallel incident rays are then allowed to fall on the reflector 

 of the microscope (either sun rays or strong white light or the 

 rays from a Nernst filament or arc light emerging at the focal 

 point of a large lens). The parallelism of the incident rays is 

 necessary and an important factor, since with the thick quartz 

 plates a slight deviation from normally incident and parallel 

 light produces disturbing interference phenomena. The 

 bi-quartz wedge-plate in its metal casing may then be placed on 

 the microscope stage and with it the accuracy of the crossing of 

 the nicols tested directly, just as the position of zero extinc- 

 tion of a mineral plate is tested. The error of such a deter- 

 mination should be considerably less than 10' of arc. 





