10 
ELEMENTARY CHEMICAL MICROSCOPY 
If, on the other hand, the investigation to be conducted 
involves much photomicrographic work, photo-objectives, apo- 
chromatics, or better still, the very carefully constructed micro- 
planars, microsummars, or microanastigmats, should be selected. 
For in addition to the fact that the chemical or actinic rays are 
not properly brought to a focus, it should be remembered that 
ordinary microscopic objectives are corrected for a fixed tube 
length, usually i6o millimeters, while in the case of photographic 
work the distance between objective and plate holder is vari¬ 
able and in all cases much greater than the standard tube length. 
THE CARE OF OBJECTIVES. 
Objectives should always be most carefully handled and pro¬ 
tected from dust and vapors. They should be kept dry and 
clean by wiping with clean new lens paper} Never use a piece 
of lens paper more than once, nor touch the lenses of objectives 
or oculars with the fingers or with cloths. 
When abrasives are employed (as, for example, in metallo- 
graphic work) even in adjoining rooms, all lenses should first be 
blown upon (but not breathed upon) and then dusted off with 
a very soft camel’s hair brush before mping with lens paper, 
otherwise serious scratching of the glass will sooner or later 
result. 
Dust on the back lens combination of the objective is often 
responsible for great loss of definition and greatly reduces the 
resolving power of an objective. Dust on the rear lens may 
easily be seen by removing the ocular, illuminating the objec¬ 
tive to its full capacity and looking into the microscope tube. 
Often a screen of ground glass placed in front of the microscope 
mirror renders the dust particles more clearly discernible. 
After using an immersion objective immediately wipe.off the 
immersion fluid with lens paper, then if the fluid is oil, wipe the 
lens with lens paper moistened with xylene, and finally wipe 
dry. Never use alcohol in cleaning objectives or any part of 
the microscope. Never allow an objective to remain moistened 
^ “ Lens paper ” is a soft absorbent tissue-like paper made from long flexible 
fibers expressly for cleaning lenses. 
