334 THE Microscope. 
conspicuously assertive. And during the use of the micrometer 
for the measurement of microscopic objects, it was a. menacing 
danger, since the value of the micrometer spaces was changed 
with every turn of the fine adjustment screw. With the mechan- 
ism in the arm, all this annoyance is done away with, since 
every movement of the screw moves the entire body, including 
the objective and the eye-piece. If properly made it has’no lost 
motion, and no side movement. It responds immediately to a 
touch of the finger, moving the body directly upward or down- 
ward with no lateral play, so that the image, even under the 
highest powers, does not seem to change its position from side to 
side, a fatal defect if present. The screw is usually and _ prefer- 
ably placed vertically at the back of the arm, within easy reach 
of the fingers as the microscopist’s elbow rests on the table, but 
some makers place it under the front of the arm, at the side, or 
even at the back but in a horizontal position or almost, at right 
angles to the optic axis, that imaginary line drawn through the 
centres of the mirror, sub-stage, stage, objective, body or eye- 
piece. 
The mechanical devices through which the screw acts on the 
tube are various and ingenious, but the effect, that of moving 
the entire body, is the same in all. I wish the method could be 
claimed as an American invention, but it can not, for it has long 
been. used in variously modified forms in England and else- 
where. It was one of the advantages of the Ross model. In 
this country, however, Mr Joseph Zentmayer seems to have in- 
troduced it, first adding it to his splendid “American Centen- 
nial ” stand, which was shown at the Centennial Exhibition at 
Philadelphia in 1876, and there formed one of the great attrac- 
tions for microscopists. On this magnificent instrument, which 
approaches perfection more nearly than any other microscope 
stand in the known world, although Mr W. H. Bulloch’s “Con- 
gress”? is not far behind it, being in some respects its superior, 
the fine adjustment is removed, as Mr Zentmayer says, to the 
more stable part of the stand, the arm, which is provided with © 
two slides, one for the rack and pinion adjustment, and close to 
it another one of nearly the same length for the fine adjustment 
moved by a lever concealed in the bent arm, and acted upon 
by a screw whose margin is graduated for the measurement of 
thin glass covers. ) 
Mr W. H. Bulloch’s “ Congress” stand, which is somewhat in- 
