448 



SDMMABY OP CURRENT RESEARCHES RELATING TO 



out of the four radii thus produced the divisions are interrupted for a 

 distance of 10 division lines from the centre. The image is thereby 

 rendered clearer in the centre. Besides these divided radii there are 

 four undivided ones, each of which bisects a quadrant. One octant also 

 is subdivided by radii into 10°, 15°, and 20°. To this division of the 

 micrometer-plate corresponds a lithographic chart, which is used in the 

 preparation of the drawing to be made. While the apparatus enables 

 an object to be measured in various directions, by the aid of the paper 



Fig. 67. 



chart it allows the sketch of an accurate drawing to be made. When 

 once from the first position five points (at the centre and on the four 

 radii) are accurately determined, the ocular is turned, so that the un- 

 divided radii pass through the points of the preparation through which 

 the divided radii previously passed. Thus four more points are obtained, 

 and if these should not suffi.ce for the sketch the radii of the octant can 

 be made use of to obtain fresh points. In a similar way angles can be 

 measured. 



Krysinski's Eye-piece Micrometer and its uses in Microscopical 

 Crystallography.* —The method proposed by Wertheim in 1862 for 



* Zeitschr. f. Krystallogr., xiv. (1888) p. 17. 



