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, 46 ELEMENTARY CHEMICAL MICROSCOPY 
ing surfaces) as to converge to an axial point lying just above 
the plane of the object upon the object slide, it is, of course, 
essential that the thickness of the object slide be known, for if 
too thin the illuminating rays will meet too far above the material 
to be studied, or if too thick the focal point will lie too low; for 
these reasons optical instrument makers mark upon the devices 
the object slide thickness to be employed. For example: 
Thickness of object slide. 
Bausch & Lomb paraboloid illuminator.. . i. 
1.40 to 1.55 mm. 
i.o to 1.10 mm. 
0.7 to 1.10 mm. 
2.0 mm. 
less than i mm. 
1.2 mm. 
1.9 mm. 
Reichert reflecting condenser. 
Reichert slip-in reflecting condenser. 
Zeiss cardioid condenser for quartz cell. 
Spencer Lens Co. Dark-field illuminator. 
Absolutely clean object slides and cover-glasses are essential 
and great care must be exercised in wiping off the immersion 
fluid from the condenser to avoid scratching the glass. Lens 
paper of the highest grade only should be employed, and the 
wiping off of the fluid should be done with the least pressure 
possible, otherwise fatty material from the fingers may be forced 
through the pores of the lens paper upon the glass. A mere trace 
of grease upon the glass surface will lead to the formation of 
air bubbles, or will prevent optical contact if water is the immer¬ 
sion fluid. Remove all traces of oil with xylene. 
The preparation to be studied must be thin and must be 
covered with exceptionally clean and very thin cover-glasses. 
Covering the preparation with a cover-glass is essential. 
In order to expedite the adjustment it is well to have at hand 
a permanent slide of some material which yields good results 
with dark-field illumination, as, for example, diatomaceous 
earth. With such a preparation on the stage the radiant, micro¬ 
scope mirror and the condenser are all so mutually arranged 
as to yield the best illumination of the diatoms; the final adjust¬ 
ment is then made by raising or lowering the condenser. The 
