46 CONSTRUCTION OF THE MICROSCOPE. 



mosum is illuminated by a Gillett's condenser, with a central stop 

 placed under the lenses, and viewed by a quarter-inch object-glass of 

 70 aperture, both being accurately adjusted, we may observe in suc- 

 cession, as the object-glass approaches the object, first a series of well- 

 defined bright dots ; secondly, a series of dark dots replacing these ; 

 and thirdly, the latter are again replaced by bright dots, not, however, 

 as well defined as the first series. A similar succession of bright and 

 dark points may be observed in the centre of the markings of some 

 species of Coscinodiscus from Bermuda. 



These appearances would result if a thin plate of glass were studded 

 with minute, equal, and equidistant plano-convex lenses, the foci of 

 which would necessarily lie in the same plane. If the focal surface, or 

 plane of vision, of the object-glass be made to coincide with this plane, 

 a series of bright points would result from the accumulation of the 

 light falling on each lens. If the plane of vision be next made to 

 coincide with the surfaces of the lenses, these points would appear 

 dark, in consequence of the rays being refracted towards points now 

 out of focus. Lastly, if the plane of vision be made to coincide with 

 the plane beneath the lenses that contain their several foci, so that 

 each lens may be, as it were, combined with the object-glass, then 

 a second series of bright points will result from the accumulation 

 of the rays transmitted at those points. Moreover, as all rays capable 

 of entering the object-glass are concerned in the formation of the 

 second series of bright focal points, whereas the first series are formed 

 by the rays of a conical shell of light only, it is evident that the circle 

 of least confusion must be much less, and therefore the bright points 

 better defined in the first than in the last series. 



If the supposed lenses were of small convexity, it is evident that the 

 course of the more oblique rays only would be sensibly influenced ; 

 hence probably the structure of P. angulatum is recognised only by 

 object-glasses of large angular apertures, which are capable of admit- 

 ting very oblique rays. 



It does not appear to be desirable that objects should be illumi- 

 nated by an entire, or, as it may be termed, a solid cone of light of 

 much larger angle than that of the object-glass. The extinction of 

 an object by excess of illumination may be well illustrated by viewing 

 with a one-inch object-glass the Isthmia, illuminated by Gillett's con- 

 denser. When this is in focus, and its full aperture open, the markings 

 above described are wholly invisible ; but as the aperture is succes- 

 sively diminished by the revolving diaphragm, the object becomes 

 more and more distinct, and is perfectly defined when the aperture of 



