246 Mr. C. Spurge. On the Effect of Polish on the 



could be restored to position by placing the diaphragms and screens 

 in their proper stations, and setting the crystal so that a bright dot of 

 light was visible to the eye in front of the last screen. 



The only possible changes in position that this method allows are a 

 displacement parallel to the table and a rotation of the face of the 

 crystal in its own plane. The former was prevented by means of 

 fixed marks on the table, and the latter by taking every precaution to 

 leave the base of the crystal in contact with the table unchanged, and 

 later on by the use of a template. 



A series of experiments were made to determine whether the dia- 

 phragms and screens could be removed and replaced in exactly the same 

 positions. For this purpose the screens and diaphragms were removed 

 one at a time and replaced, using only the setting lines. It was found 

 that the dot of light remained visible, while the slightest displace- 

 ment of the screens caused it to disappear. 



During each set of experiments the screens and diaphragms were 

 frequently replaced to determine if the crystal remained unmoved. 



The surface of the crystal was shielded during the day by a box 

 with apertures, and completely covered at night. In taking the 

 readings the quarter wave plate and Nicol were turned into snch 

 positions that the centre of the field was as dark as possible. 



IV. Experiments made with a Natural Face. 



Observations were now made with the light reflected from that 

 natural face of the crystal which seemed the best. The results are 

 given in Tables I, II. The observations made with the crystal were 

 always consecutive, none being rejected after the first satisfactory 

 observation had been taken. 



Table I. — Observations with a Natural Face. Mean Temp. 15°"3 C. 





r'. 



r. 



E'. 



E. 



(/-r). 



(E'-E). 





90-562° 



4-noi° 



153 '895° 



62-475° 



85 -961° 



91 -420° 





90 -585 



4 '565 



153-781 



62 -286 



86 • 020 



91 -495 





90-350 



4-334 



153-955 



62-256 



86 016 



91 -699 





90-293 



4-324 



154 -002 



62-516 



85-969 



91-4S6 





90 -285 



4-525 



153-840 



62-462 



85 -760 



91 -378 





90 -454 



4-594 



153-852 



62 314 



85 -860 



91-538 





90-278 



4-430 



153 -790 



62-408 



85-848 



91 -?82 





90 '362 



4-321 



153-888 



62 -315 



86 -041 



91-573 





90 -486 



4-470 



153-866 



62-339 



86 -016 



91 -527 





90 -045 



4-525 



153 833 



62 -215 



85 -520 



91 -618 





90 -287 



4-545 



153-433 



62-317 



85 742 



91-116 





90 -370 



4-470 



154 -020 



62 -264 



85 • 900 



91 -756 





90-312 



4-450 



153-687 



62-285 



85 -862 



91 -402 



Means . . 



90 359 



4 473 



153-834 



62-342 



85 -886 



91-492 



