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 160 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. 1 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 wiping 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 



1 " Lens paper " is a soft absorbent tissue-like paper made from long flexible 

 fibers expressly for cleaning lenses. 



