116 HOW TO SEE WITH THE MICROSCOPE. 



stance, one -sixth are now made w r ith apertures we will 

 say (to keep out of controversy) up to 180, and with a 

 working distance of one-fiftieth of an inch. Now the 

 widest angled one-fiftieth in existence, with a working 

 distance le^s than half that of the one-sixth, will be 

 found to measure less than 170, and in the latter glass 

 it is evident that, working distance has been sacrificed 

 without corresponding increase of angular aperture. The 

 case cited is an instance notably in point, and one that 

 cannot be dodged, and yet to a certain extent the same 

 will apply to some of the intermediate objectives. 

 Take again the before-mentioned sixth of the widest 

 angle known, and its working distance of one-fiftieth of 

 an inch. It will be found that, although it is possible 

 to obtain the same aperture for the 1 tenth, the working 

 distance will suffer decrease ; and here again is another 

 instance where sacrifice of working distance is not 

 accompanied by corresponding increase of aperture. 



The subject is by no means exhausted, and is well 

 worth a little ventilation. We can better get at the 

 situation by supposing a case which might possibly 

 occur in practice. Suppose then, reader, that you 

 desire five one-inch glasses, each glass to have a work- 

 ing distance of five-tenths of an inch, and each to mag- 

 nify with the two-inch ("A") eye-piece, fifty diameters, 

 and that you gave these five glasses respectively to five 

 opticians, to be made as per the conditions named. 

 Now, it will most probably occur that when you get 

 these five glasses in hand, the working distance and the 

 magnifying powers of each are true to the specifications, 



