118 IlOW TO SEK WITH THE MICROSCOPE. 



applied to advantage to wit: we can send to Mr. Y. 

 for another inch similar to the one in hand, but with a 

 lower aperture, and corresponding to that of Mr. X., 

 and it will obtain that Mr. Y., in cutting down the 

 aperture of his inch to that of Mr. X., will increase his 

 working distance; and here (comparatively) we gain 

 working distance without loss, or, as it has been termed, 

 sacrifice of angular aperture. 



Hence we arrive at the conclusion that the function 

 recognized as "angular aperture " per s>i is not a fixed 

 and definite quantity nor one that can be fenced in and 

 subjected to any fixed rules. Nothing definite in the 

 way of rigid law can be applied to it. In the case just 

 mentioned, another curious conclusion might be arrived 

 at, and justly too. For instance, the decrease of aper- 

 ture from that of the wider aperture to that of the lower 

 would not only be accompanied (accepting the popular 

 dogma), which in the case in question would hold true 

 by an increase in the working distance, but the penetra- 

 ting power of the glass would thereby be enhanced, and 

 this, too (comparatively), without loss of angle. 



The facts presented are valuable, are significant, and 

 worth careful thought and study. The author has 

 never seen them in print, and they are, as suggested, 

 the result of an active experience. 



And this brings us to the consideration of another 

 matter; I refer to the popular dogma of * 'penetration. " 

 This has been the biggest toad in the puddle, and has 

 exercised an active agency in roiling and mystifying the 

 mind of the microscopist. The doctrine of penetration 



