of Minerals in the Thin Section. 331 



Mallard's method for measuring the optic axial angle is one 

 of the most satisfactory of the microscopic methods and if 

 available sections are at hand which show the required phe- 

 nomena, Mallard's method should be adopted without question, 

 especially if the measurements can be made with a double 

 screw micrometer ocular. The limits of error of measure- 

 ments of 2Y by the Mallard method should not exceed l°-2° 

 on clear interference figures. 



Methods of F. Becke. — In place of the single screw microme- 

 ter ocular which in itself is of very limited application, F. 

 Becke* has substituted a graphical method in which the 

 observed optical phenomena are projected by a camera lucid a 

 on a revolving drawing table fixed in position relative to the 

 microscope. Accurate drawings of the interference phenomena 

 are thus prepared and serve in place of the actual interference 

 figure. This method has been fruitful in its results and with 

 practice the necessary manipulative skill can be acquired to 

 obtain trustworthy axial angle values. The accuracy of the 

 method is dependent on several factors — the accuracy with 

 which the drawing is prepared, the exactness with which the 

 drawing table is centered, and the care with which measure- 

 ments are made on the finished drawing. 



The actual field of the projection does not measure over 

 25 mm in diameter, and a difference of 1° of E corresponds to a 

 difference in D of about •25 mm i a distance which is easily meas- 

 urable. With unusually sharp axial bars and nice adjustment 

 of the optical system, it is theoretically possible to obtain an 

 accuracy of about -J— |- ; in practice, however, a greater 

 accuracy than 1-2° cannot be claimed for the method. 



The writer has not seen the revolving drawing table described 

 by Professor Becke, and has used in his work a small revolving 

 disk D, graduated in degrees and supported by an arm E 

 attached to the microscope stand by the collar F, as shown in 

 fig. 7. This device was constructed in the mechanical work- 

 shop of the Geophysical Laboratory, after specifications fur- 

 nished by the writer. The results obtained with it have proved 

 satisfactory and the manipulation with the same convenient. 



Having once fixed the position of this table so that its axis 

 of revolution coincides after reflection in the camera lucida 

 with the optical axis of the microscope and is also at the 

 proper distance from the eye for distinct vision, its constant K, 

 corresponding to the K of the microscope in the formula 

 D = K sin E, must be determined by one of the methods 

 described above. 



With the drawing of an interference figure thus properly 

 prepared, it is possible to determine the angular distance, — polar 



* F. Becke, Tschermak'sMin. petr. Mittheil., xiv, 563, 1894 ; xvi, 180, 1896. 



