60 CONSTRUCTION OF THE MICROSCOPE. 



lens, it is difficult to conceive why the oblique rays only 

 should be visibly affected. When P. hippocampus or P. for- 

 mosum is illuminated by a Gillett's condenser, with a cen- 

 tral stop placed under the lenses, and viewed by a quarter- 

 inch object-glass of 70 aperture, both being accurately 

 adjusted, we may observe in succession, 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 accumu- 

 lation 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 admitting 

 very oblique rays, 



It does not appear to be desirable that objects should be 



