540 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
plate which permits a lateral movement of the frame. This lateral 
motion is controlled by a screw, operated by means of a second milled 
head in convenient proximity to the one giving the to-and-fro motion. 
The advantages of this form of apparatus are that ordinary glass slides 
may be used, and that the focal distance is not disturbed by differences 
in their thickness. The only disadvantage is that the mechanism is 
somewhat more delicate and complicated than in the other instrument, 
and requires some little protection from the thoughtless violence of 
Fig. 134. 
curious children. Microscopes copied from the original have been in 
use for several years and have proved perfectly satisfactory. 
The writer in the Scientific American describes a valuable im- 
provement intended to prevent injury to the instrument from violent 
twisting of the milled head, which controls the lateral movement of the 
frame after the frame has been brought up against the stops in either 
direction. This is effected by slightly tapering the pivot of the screw 
governing the movement, and attaching the head by friction only, the 
amount of friction being regulated by a set-screw in the end, so that, 
