18 MICROSCOPY FOU BEGINNERS. 



(several opticians' "Students' Series" include them), the 

 working distance is not too short, or need not be, and 

 with the two-inch eye-piece it will give a magnifying 

 power of about two hundred and fifty diameters. 



"The coarse adjustment" is the expression usually 

 applied to the rapid movement of the body produced 

 by turning the large milled heads, one of which is on 

 each side of the instrument. It is used in focussing, 

 that is, in obtaining a distinct image of the object when 

 seen through the eye-piece and objective. The image 

 then appears surrounded by a circle of light called the 

 "field of view," or simply "the field." Yery few, ex- 

 cept the small, vertical "boys' microscopes," and some 

 of the cheapest and least desirable American or English 

 stands, are without the coarse adjustment. Occasionally 

 a stand will be seen in which this part is replaced by a 

 broad, cloth-lined, or tightly-fitting collar, through which 

 the body slides, the movement being made by hand. 

 This is very unsatisfactory, and such stands should be 

 avoided, if possible, as, sooner or later, the body is sure 

 to be suddenly pushed too far down, the objective then 

 coming in contact with the object : an accident to be al- 

 ways guarded against with the greatest care, as the ob- 

 jective, or the object, or both, may be injured. If the 

 object is destroyed it may possibly be replaced, but a 

 scratched or broken objective can be remedied only by 

 buying a new one. Of course the microscope body may, 

 by a careless student, be forced against the object by 

 the use of the milled heads, and equally, of course, a 



