MICROSCOPES. 211 



the width of those in the case of a dry lens of equal aperture, 

 and therefore, other things being equal, an immersion lens 

 may be said to have four-thirds the defining power of a dry 

 lens. In all these kinds of examinations, in order to fully 

 utilise the capabilities of the object-glass, the light passing 

 from the object ought to be of the same angle of divergence 

 as that which can enter the object-glass. With an object 

 not covered you might obtain greater divergence, if light 

 could be made to pass in at a greater angle. However, in 

 making object-glasses of very wide aperture we come across 

 great difficulties. By increasing the size of the aperture we 

 increase the difficulty in making them correct in other respects, 

 especially in correcting the spherical and chromatic aberrations. 

 Another practical difficulty is that the object-glasses come 

 down so close to the object, and it is impossible to see any- 

 thing through a thick cover, or when some distance within 

 a transparent portion of the object itself, as is so often the 

 case in studying the microscopical structure of minerals, or 

 the crystals enclosed in blowpipe beads. I very much fear 

 that it is only too true that if you improve object-glasses 

 in one respect, you make them worse in another. Those 

 qualities which are necessary for one purpose are unnecessary 

 for another ; and in my opinion we ought to have object- 

 glasses constructed for the particular kinds of work we wish 

 to use them for. If we wish to define very close markings 

 on thin flat objects, like diatomaceae or the minute striae 

 of muscular fibre, we must have a very wide aperture, even 

 if the object is then so very close to the object-glass that the 

 range of vision is too small when we come to study other 

 kinds of objects. For instance, in some cases, with the very 

 best object-glasses of a very high angle, which would give 

 the most splendid definition of test objects, you might not 

 be a"ble to see other objects you wanted to examine, because 

 you could not get at them. They would be beyond the focal 

 point when the object-glass touched the cover. In such a 

 case, with only moderately fine structures, an object-glass of 

 much less cost and smaller angle of aperture might enable 

 you to see all the necessary detail, and you could get at the 

 object, even when under one-tenth of an inch of Canada 

 balsam, or other transparent substance, where you could not 

 have seen it at all with a lens of high angle and of the same 

 magnifying- power. I am, therefore, inclined to believe that 



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