THE MICROSCOPE AND ITS PARTS. 
21 
style of adjustment, but in photo-micrography all our 
experience leads us to prefer the entire tube arrange¬ 
ment. 
Whichever of the systems we choose, great care 
should be taken to make sure that the fine adjustment 
works straight, without the least side motion in its 
course; that it “loses no time,” i.e., that it answers 
to the least touch on the screw by which it is manipu¬ 
lated ; that its motion is sufficiently slow, and that it is 
Fig. 2. 
absolutely free from a tendency to “ jump ” forward or 
backward. The coarse adjustment is usually worked 
by a rack and pinion ; but there is a vast practical 
difference between a good rack and a poor one. The 
rackwork should be diagonal and not perpendicular 
to the axis of the tube (fig. 2). This diagonal arrange¬ 
ment is found in almost all of the best stands of the 
present day. As pointed out in Dr. Dallinger’s edition 
of “Carpenter,” the groove, of whatever shape, in which 
the bar of the coarse adjustment works, should be cut 
and sprung, so that when the fittings work loose, as in 
