USE OF I'liK HAM) m.u;nifier 



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the most important figure connected with the microscope. 

 There can be no understanding of the microscope without 

 understanding aperture. The aperture of a magnifying lens 

 larger than the pupil depends on the diameter of the 

 pupil of the observer's eye (Fig. 4). Thus both increase 

 in a dull light. (This does not usually happen in the 

 eyepiece of a microscope, which is, of course, a magnifying 



Fig. 4. — Diagram of eye and low-power corrected lens, to show field of view. 

 Best vision is obtained when the eye is centered with the magnifier, as here. 

 Three cones of rays (bounded by the iris) are shown from points at the center 

 and edges. It is obvious how moving the eye farther from the lens will decrease 

 the field of view. 



lens combination, because the pupil of the eye is usually 

 larger than the exit circle at the eyepoint of the microscope.) 

 Since the aperture of a lens used as a magnifier also depends 

 on its focal length, which is inversely proportional to its 

 magnification, the brightness of an object seen through a 

 corrected lens larger than the pupil is equal to the brightness 

 as seen with the unaided eye; except for a loss of 8 per 

 cent, more or less, in reflection at the two surfaces. (An 

 increase in magnification of the eyepiece of the compound 

 microscope, on the .other hand, mostly leads to a marked 

 decrease in illumination, which has to be compensated for 

 by lessening the density of the screen before the source of 

 fight.) 



