34 ELEMENTS OF APPLIED MICROSCOPY. 
tion may be distracted by external objects, but this difi- 
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 
muscz 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 qualities 
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- 
