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movement upon the stand. Tlie lens can tlius be raised or 
turned in any direction, tlie intention being so to adjust it as 
to concentrate the rays in a focus upon the opaque object 
under examination. The plane surface should be nearest to 
flame. 
The position which the observer assumes should be one 
which is consistent with his own ease and comfort. In a 
constrained attitude he will soon tire, and physical fatigue will 
be followed by loss of interest and power of observation. The 
table, therefore, as well as the seat, should be regulated to meet 
his requirements, and such an obliquity given to the body of the 
microscope as may suit both. In looking for a long time down 
an instrument, the eye is liable to become fatigued, especially 
if the same eye is always used ; it is desirable, therefore, that 
the observer should learn to use either eye at pleasure, and 
occasionally to change one for the other. But, since Ave are 
aware, from experience, that this is not always an easy matter, 
it will be well early to accustom oneself to keeping the 
unemployed eye open, by which means the fatigue arising from 
screwing it up with a constrained and constant effort, is 
avoided.* 
The object-glasses A r ary in magnifying capability, and are 
hence called powers. Those whose focal distance is gTeater 
* On no account should the observer continue the inspection if he feels pain 
or fatigue. Many microscopists have either impaired or destroyed their 
eyesight by too long continuance at the instrument, or by its too frecpient 
employment. 
