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 microscopic 

 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 i& 

 enabled to see with the eye-glass the magnified image of the 



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