50 THE ELEMENTS OP VEGETABLE HISTOLOGY 



For a few minutes, there will be difficulty in keeping 

 both eyes open and in concentrating so that one obtains 

 a clear picture with the eye used for observation, but 

 this difficulty is very soon overcome. Still more dif- 

 ficult, but equally necessary, is the ability to use 

 each eye for observation and to be able to change 

 rapidly from one to the other. The proper distance 

 of the eye from the ocular varies with oculars of dif- 

 ferent powers. The eye-point for low-power oculars 

 is at a greater distance from the eye lens than is the 

 case with those of high power. Cleanliness of lenses 

 and specimens, together with the proper regulation 

 of light, are important factors in saving the eyes from 

 undue strain. 



Cleaning the Microscope. — Dust particles and 

 finger marks cause serious difficulty and annoyance 

 in working with a microscope. These foreign materi- 

 als are magnified just as much as the specimen and 

 thus result in indistinct images. Finger prints upon 

 optical parts are a sure sign of the careless worker, 

 and as they are easily avoided are all the more aggra- 

 vating. -Slides and cover-slips should always be 

 handled by their edges, and the lens surfaces should 

 never be touched with the fingers. Finger prints 

 may be removed by breathing upon the surface show- 

 ing them, and quickly wiping it dry with cloth or 

 lens paper. Old smears not yielding to this treatment 

 may be removed with a cloth sparingly moistened with 

 alcohol or xylol; but the solvent must not be allowed 

 to penetrate the lens mounting. Dust particles on 

 lenses may be removed with a camel's hair brush or 

 dry soft cloth. By rotating the condenser, objective 

 and ocular, the position of dirt or other interfering 



