FIELD OF VIEW. 
The entire dimensions of the field of view therefore being so 
exceedingly minute, it will be easily understood that some diffi¬ 
culty will attend the process by which a small object, or any 
particular part of an object, can be brought within it: thus, with 
a moderate power of 500, the entire diameter of the field being no 
more than the 70th of an inch, a displacement of the object to 
that extent, or more, would throw it altogether out of view. If 
therefore the object, or whatever supports it, be moved by the 
fingers, the sensibility of the touch must be such as to be capable 
of producing a displacement thus minute. 
If the object be greater in its entire dimensions than the field 
of view,—a circumstance which most frequently happens,—a part 
only of it can be exhibited at once to the observer; and to enable 
him to take a survey of it, it would be necessary to impart to it, 
or to whatever supports it, such a motion as would make it pass 
across the field of view, as a diorama passes before the spectators, 
disclosing in slow succession all its parts, and leaving it to the 
power of the observer to arrest its progress at any desired moment, 
so as to retain any particular part under observation. 
The impracticability of imparting a motion so slow and regular 
by the immediate application of the hand to the object, or its 
support, will be very apparent, when it is considered that while 
the entire object may not exceed a small fraction, say, for exam¬ 
ple, the 20th of an inch in diameter, the entire diameter of the 
field of view may be as much as 20 times less, so that only a 20th 
part of the diameter of the object would be in any given position 
comprised within it. 
23. These and similar circumstances have rendered it necessary 
that the want of sufficient sensibility and delicacy of the touch in 
imparting motion to the object, shall be supplied by a special 
mechanism, by means of which the fingers are enabled to impart 
to the object an infinitely slower and more regular motion, than 
they could give it without such an expedient. The means by which 
this is accomplished will be presently explained. 
We have seen that the intensity with which the microscopio 
image is illuminated depends on the angle of aperture, other 
things being the same ; but however large that angle may be, 
when considerable magnifying power is used, it is necessary that 
the object itself should be much more intensely illuminated than 
it would be by merely exposing it to the light of day, or that of 
the most brilliant lamp. It is therefore necessary to provide 
expedients, by which a far more intense light can be thrown 
upon it. 
24. The instrument is said to be in focus when the observer is 
enabled to see with , the eye-glass the magnified image of the 
27. 
