OF THE MICROSCOPE. 49 



objects, and the figures on the preceding page were used by him 

 to define and explain what he meant by angular aperture. 



In these figures L L and L' L' are two lenses of the same 

 magnifying power, but different angular apertures. It will be 

 seen that the cone of rays proceeding from O, is substantially 

 the same as that from O', but that the lens L L takes in a larger 

 part of the cone from O, than the lens L' L' does of the cone 

 from O'. The angles L O L and L' O' L' are the respective 

 measures of the angular apertures of the two lenses. 



The definition of angular aperture given by Goring, has 

 been followed by all subsequent writers, the accompanying figure 

 being that used by Dr. Carpenter for 

 the purpose of explaining and de- 

 fining the same thing. In this figure 

 <( b c is the angle of aperture. 

 Therefore, while it is not to be denied 

 that other angles may be of great im- 

 portance in considering the qualitic s 

 of an objective, it is altogether wrocg 

 to apply the term angular aperture to 

 any other angle than the one that has 

 been described. 



Dr. Goring devised several practi- 

 cal methods of measuring the angular 

 aperture of different objectives, and 



he gives a very full and clear description of an arrangement 

 adapted to his own instrument, in which the foot was made to 

 rotate on a carefully centered and graduated base a very ex- 

 cellent plan, modifications of which have since been adopted 

 by Mr. Zentmayer, of Philadelphia, and by Mr. Bulloch, of 

 Chicago. 



It is not difficult to demonstrate the importance of a compara- 

 tively wide angle of aperture, since object-glasses possessing 

 this feature are capable of giving important results which 

 lenses of lower angle cannot give. Thus, when we examine, by 

 means of a superior French triplet of one-sixth of an inch 

 focus, the silicious remains of certain very minute plants of the 

 species Pleurosigma Balticum, we are able to see certain lines or 

 markings which exist upon their surfaces. That we may be able 



