702 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



crystal the edge and tlie thread must remain parallel. (2) The edge 

 is so centered that it appears in the middle of the cross-threads of 

 the telescope, and remains in this position on turning the crystal, 

 (3) The screw H is turned to fix the lower slide of the Microscope, 

 so that the latter can only be moved at right angles to the gonio- 

 meter-axis. The Microscope is focused on one of the two surfaces F G 

 (fig. 127), forming the edge h to be measured, and having focused the 



part next to k, the Microscope is passed over the surface G, and the 

 crystal slightly moved until the part next i is in focus. When G has 

 been adjusted exactly horizontal, the crystal is then turned, and the 

 surface F similarly adjusted. The angle indicated on the goniometer 

 through which the crystal is turned in order that the second surface F 

 may occupy the position of the first, G, is the complementary angle 

 of the edge measured. 



The degree of exactness of the measurements for a diameter of the 

 crystal surface of x mm., and for a defect in the focusing of the 



Microscope of u mm., will be given by the formula tan a = 2 — . 



Therefore for 



ic = 10 mm. and u = O'OOd mm., a = 2' 45" 

 = 5 =5' 30" 



a; = 10 mm. and w = • 008 mm., a = 5' 30" 

 = 5 =11' 0" 



These figures, however, involve extreme assumptions : by careful 

 repetition of the measurements exactness to 1' and less can be 

 secured. 



It is recommended to dust the surface of transparent crystals with 

 very fine lime-wood charcoal, the focus not, however, being adjusted 

 to the grains of charcoal, but upon the surface on which they rest. 

 For opaque objects fine gum arable is best. 



