470 
POPULAR SCIENCE REVIEW. 
ment, it must be moved backwards and forwards until the true 
focus is found, and consequent clear definition is obtained. 
But tbe fine adjustment lias other uses besides merely finding 
the focus with a bigb power ; with the finger upon it, and the 
eye at the microscope, the object should be well scrutinized, 
and each part or stratum of its thickness successively focused, 
the relations of these strata to one another only being satis- 
factorily determined while in the act of changing the focus. 
In using the coarse adjustment with high powers, the great 
thing to be avoided is heedlessly bringing down the object- 
glass upon the slide under examination, which an incautious 
turn of the screw may too easily accomphsh, and which makes 
possessors of good microscopes very careful how they allow 
strangers to meddle with then’ instruments. The result of 
this accident may be either to break the glass slide on the 
stage, and totally destroy what is perhaps a valuable object, — 
or worse, it may result in a chip or scratch of a ten-guinea 
objective. 
The young microscopist is very liable to fall into considerable 
errors of judgment while examining an object, particularly under 
high powers, arising from his inexperience and consequent in- 
ability to comprehend a distortion or recognize the truth. Nor 
need he lose courage at this, for experienced observers have 
fallen into similar errors of much more importance than any he 
is likely to make — errors which have given rise to false theories, 
and retarded the progress of science. Some of these errors, 
arising from optical appearances, can only be corrected by prac- 
tice, or by the use of more complicated apparatus which the 
student will apply in the course of time. There are others, how- 
ever, concerning which some hints may here be given. If the 
parts are not accurately in focus, the outlines may be either 
distorted, indistinct, or misrepresented. Lights and shadows may 
be completely reversed, as in the markings of the diatoms,* a 
circumstance which arises from the object itself refracting light. 
And the higher the power used, the greater is the liability to 
fall into this error. Sharply-defined black rings with a bright 
centre cause no little surprise to the beginner, which, however, 
are nothing more than minute bubbles of air. Oil-globules hi 
water present much the same appearance ; as also do globules 
of water, surrounded by a layer of oil. But if the object-glass 
be approached nearer to them, the water-bubbles and oil- 
globules get a darker centre, the water-globules in oil a fighter 
centre, and the reverse when the objective is raised. Such 
changes as these, as means of recognizing different substances 
often met with in the lower forms of animal and vegetable life. 
