20 
PRACTICAL PHOTO-MICROGRAPHY. 
the most delicate machinery of the whole instrument 
this considerable weight ; the result would seem to be 
certain damage to the adjustment in a short time. 
According to the writer’s experience, many instruments 
of Continental make, fitted with this kind of fine adjust¬ 
ment, have given way under the strain, and are value¬ 
less as instruments of precision. Further, the instru¬ 
ment usually acknowledged to be the best that is 
made, viz., that of Messrs. Powell and Lealand, has the 
nose-piece arrangement. But it must be added that at 
least two instruments, with fine adjustments actuating 
the whole tube, have stood the test of hard work and 
time in the writer’s hands, viz., the “ Campbell differ- 
en tial screw ” in Mr. Baker’s “ Nelson ” stand, and the 
modified Zentmayer lever in the “ Edinburgh ” stand, 
of Messrs. Watson & Son. There is also the “ side 
lever ” arrangement of Messrs. Swift, of which we have 
good accounts from experts, but no experience of our 
own. 
For use with the higher power “ apochromatic ” 
objectives of Zeiss, which are probably the best objec¬ 
tives to be obtained for our work, the nose-piece fine 
adjustment arrangement has the serious, if not fatal, 
fault that in use it alters the length of the tube ; and 
the “ correction ” for these objectives depends on, and 
is regulated by, this length. At the same time, it must 
be admitted that the variation in length produced by 
the working of the fine adjustment is very small, and, 
except in the most critical work, may be overlooked. 
For general microscopy we might prefer the nose-piece 
