MICROSCOPIC OBJECTS. 23 



remarks on Angular Aperture. Sir William Herschel 

 discovered that the space-penetrating power of a tele- 

 scope depends upon the diameter of its object-glass or 

 speculum. Hence arises the power of a telescope to 

 exhibit to us celestial objects which the unassisted eye 

 cannot perceive. This power is distinct from magnifying 

 power, though associated with it ; in the same manner 

 as the strength of a beam is not necessarily in proportion 

 to its weight, though we cannot have a beam without some 

 weight. Different materials of the same weight may have 

 different degrees of strength. The penetrating power 

 arises from the number of rays of hght from a given 

 object, which are condensed on the retina of the eye. 

 Suppose that the pupil or aperture of the human eye be 

 one eighth of an inch in diameter, and the object-glass 

 or metal of a telescope be eight inches, the proportional 

 diameters of the eye and telescope will be as 1 to 64. 

 Now as the areas (surfaces) of circles are to each other 

 as their squares, it follows that for every pencil of rays 

 which enters the eye, 4096 will be received by the tele- 

 scope. Now, if the celestial object be so distant that 

 fewer pencils of rays than those received by the telescope 

 are too faint to give an impression on the eye, in such a 

 case a star would be seen in the instrument while it 

 would be invisible to the eye. The same observations 

 apply to the microscope ; and the superiority of the 

 modem improvements on that instrument does not arise 

 from increased magnifying power, but an augmentation 

 of penetration ; indeed, in comparing two microscopes of 



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