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Transactions of the Society. 



for between the diameter of the lens and its N.A. in use there is, 

 generally speaking, no relation whatever. Fig. 86 illustrates this 

 point. 



Fig. 80. 



Here the limiting aperture of the system is the diameter of the 

 iris, and it is the function of the glass to deliver a beam of plane 

 wave-fronts, i.e. a parallel beam of light, to the eye. Absolutely 

 parallel it need not be, for the eye has a certain power of accom- 

 modation enabling it to supplement the work of the Microscope — 

 or other lens — to a small extent. But it is just because the inci- 

 dent beam is divergent beyond the power of easy accommodation 

 that recourse is had to the use of spectacles. 



Let it be next supposed that in place of a pair of spectacles 

 used to look directly at the ultimate object we have the eye-lens 

 of a Microscope, and view through it the image formed by an 

 objective in the image plane of the instrument. Then we have a 

 compound Microscope, and the magnifying power no longer depends 

 upon the focal length of the eye-lens or upon its N.A. as deter- 

 mined by the aperture of the pupil, but upon this multiplied by 

 the magnifying power of the objective which produces the enlarged 

 image in the image-plane of the instrument. It may seem at first 

 sight as if this were in no way limited by the diameter of the 

 pupil, but the following diagram makes it plain that the limitation 

 still holds. 



Fig. 87. 



It will appear from this without discussion that the optical 

 projection of the iris upon the principal plane of the objective is 

 the effective working aperture of the instrument, whatever may be 

 the aperture with which it is credited in the maker's catalogue, 



