PARTS OF THE MICROSCOPE 27 
is in its place before starting the micrometer thread. In some cases 
this pin even drops out of place before the micrometer thread is entirely 
out of its bearings and the defect is not noticed until the fine adjust- 
ment fails to respond. In such a case the top of the fine adjustment 
must be opened to secure the pin and put it in place. This ought to 
be done by the maker or experienced mechanic. 
In some cases, especially in microscopes (Continental type) where 
the prism is used in the fine adjustment, the lubricant in the prism 
becomes gummed so that the adjustment fails to respond promptly 
and then jumps. On the best microscopes provision is now made 
against this. The bearings should be thoroughly cleaned and oiled 
with paraffin oil or watch oil. This ought to be done by the manu- 
facturers, because the mechanism is so delicate that even though safely 
taken apart it would be put together and adjusted with great difficulty. 
All the modern microscopes have their fine adjustments so arranged 
that the fine adjustment ceases to work when the objective rests on 
the cover glass. This feature should be insisted upon 
Draw Tube. This should work easily and smoothly and should 
be kept clean and dry. In sliding this draw tube, do so with a revolving 
motion. The draw tube should always be drawn out to give the proper 
focal length for which the microscope was standardized. AIl micro- 
scopes give best results if used with cover glasses of definite thick- 
nesses. 
Substage. This should be kept dry and clean to prevent cutting 
and rusting of the delicate bearings. Should the leaves of the iris 
diaphragm become rusted, they may be cleaned with xylol and oiled. 
When they become bent, an expert is necessary. 
Lenses. Absolute cleanliness is necessary with the lenses. Japa- 
nese lens paper of highest quality should be used for cleaning. It is 
best to use no other material on the lenses. Great care should be used 
in cleaning them since they are very easily marred and scratched. 
Objectives. If these become soiled rub with lens paper using 
a little chloroform if necessary. Hard rubbing must be avoided. 
The immersion objective should always be cleaned by wiping with 
lens paper. If the oil is old and tends to be hard, xylol may have to 
be used. 
All dry objectives are corrected for a definite thickness of the cover 
glass. (Bausch & Lomb, 0.18 mm.; Spencer Lens Co., 0.18 mm.; Zeiss, 
15 mm.: Leitz, 0.17 mm.) The objectives are corrected to a certain 
tube length, which is 160 mm. by most makers and 170 mm. by Leitz. 
Such tube lengths should always be used for which the objectives have 
