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by the writers of the last few years, that it is quite unnecessary now 

 to go into the subject. 



"It is scarcely requisite to say that, in the compound micro- 

 scope, as in the telescope, the object-glass forms an aerial picture of 

 the object under examination, which picture is examined by the eye- 

 piece, the prominent difference being that with the telescope — the 

 subject being remote, and the rays of light from it being approxi- 

 mately parallel, the image is formed nearly in the principal focus 

 of the object-glass, and is smaller and proportionably more lumi- 

 nous than the object ; while in the microscope, the object being 

 near to the focal point of the object-glass, the image is formed in the 

 conjugate focus, and is considerably larger and proportionably less 

 luminous than the object itself. With both instruments the ob- 

 server sees the object by the rays of light passing from it through 

 the object-glass, — directly, if it be a luminous body, — by reflection 

 at its surface from the source of light, if it be opaque, — or finally, 

 if it be translucent, by transmission through its substance ; but I 

 think it will not be disputed that in all cases the origin of the light, 

 which is the means of vision, is the points which are seen. 



" Now, as the clearness of vision depends on the quantity of light 

 which reaches the eye, it is manifestly important that the object- 

 glass should have an aperture as large as possible; and here 

 the modern object-glass, by correction of aberrations, spherical as 

 well as chromatic, lends its wondrous aid. 



" There is, however, one wide difference between the object- 

 glasses of telescopes and of microscopes : — in both, the penetration, 

 that is, the development of minute features depends on the quan- 

 tity of light admitted, but in the former, which are corrected for 

 incident light, nearly parallel the quantity depends on the diameter 

 of the object-glass, irrespective of the distance of the object. In 

 the microscopic the corrections are for divergent light, and as 

 the quantity admitted with a given aperture is largely increased by 

 bringing the lens closer to the origin of light, the correction 

 of the aberrations for divergent pencils, especially those of such 

 extraordinary divergence as are now used, complicates the prob- 

 lem immensely ; and to this, no doubt, it is owing that so long an 

 interval of time elapsed after the achromatic object-glass of the 



