34 ELEMENTS OF APPLIED MICROSCOPY. 



tion may be distracted by external objects, but this diffi- 

 culty will pass with practice. The beginner may also be 

 misled by seeing through the microscope certain cloudy 

 specks floating across the field of view. These are the 

 muscae volitantes, shreds of matter lying in the vitreous 

 humor of the eye; after a time they are so discounted 

 by the observer that he becomes unconscious of their 

 presence. 



6. Qualifications of a Good Microscope. — The princi- 

 pal parts of the compound microscope have been de- 

 scribed in Chapter I; it remains only to point out some 

 of the most important qualities by which we may measure 

 the value of any individual instrument. Differences in 

 the mechanical parts are largely a matter of personal 

 preference, although the stand should be as compact as 

 possible with a base sufficiently heavy to give proper 

 steadiness. Both the coarse and fine adjustments must 

 work easily without being so loose as to allow the tube to 

 slip down of its own weight. The fine adjustment should 

 have an ample excursion. The mirror-bar ought to 

 move freely, and yet retain any position in which it is 

 placed. The stage should be wide enough to accommo- 

 date large objects when necessary. 



The optical parts are, of course, of prime importance; 

 they should be examined with respect to the four quahties 

 of magnifying power, resolving power, penetration, and 

 illuminating power. The magnification of a lens, or the 

 ratio between the size of an object and the size of the image 

 formed, depends simply on the curvature of its surfaces 

 and may be measured by the method described in Chap- 



