THE APEKTCRE OF THE OBJECTIVE. 60 



latter again replaced by bright dots, not, however, so 

 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 when viewed by transmitted light. 



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 sur- 

 faces 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 accumula- 

 tion of the rays transmitted at those points. Moreover, 

 as all rays capable of entering the object-glass are con- 

 cerned in the formation of the second series of bright 

 focal points, whereas the first series being 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. 



The Aperture of the Object-glass. The aperture of 

 an objective has been, down to a comparatively recent 

 period, the occasion of much controversy. It was 

 contended that the aperture of a dry objective of 180 

 angle represented the largest aperture possible, that 

 this could not be exceeded by any immersion objectives, 

 the advantages of the latter resting solely upon the 

 increase in light, through the absence of reflection at 

 the surface of the front lens, and their greater working 

 distance. 



The confusion into which the aperture question was 

 brought by this contention, arose almost entirely from 

 the fact that its supporters had not appreciated the 



