392 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
latter is held firm by the clamp at D, and on removing the upper part 
can be taken out for cleaning. Backlash of the micrometer-screw is 
completely avoided by the spring fastened at G, which extends to E. 
Fio. 4.'). 
On the micrometer-screw is the drum H divided into 100 parts, and the 
head I for clamping ; the sharp end of the piece K screwed to the socket 
of the eye-piece serves as index. The upjier lens of tlie eye-piece can 
be drawn out (B, fig. 45) for the exact focussing of the micrometer' 
Fig. 46. 
divisions. The whole is fixed to the body-tube by the screw 0, tliougli 
a slight shaking of the body-tube, owing to the movement of the micro- 
meter-screw, docs not injuriously affect the exactness of the measure- 
ments. 
(5) Microscopical Optics and Manipulation. 
On the use of Fluorite for Optical Purposes.* — Prof. E. Abbe lias 
an article on this subject. Of the minerals which occur in nature, 
quartz and calc-spar are the only two which have been regularly used in 
Zeitschr. f. lustrumentenk., x. (1890) pp. 1-6. 
