USE OF THE APERTOMETER. 237 



Abbe's apertometer is one of the instruments which may 

 be profitably purchased by the executive of every micro- 

 scopical society, for use ia testing the apertures of the 

 members' objectives. Such a proceeding would often save 

 the individual member much annoyance, and he would 

 then be certain of what he was purchasing. A case of this 

 kind has just fallen under the author's notice. A \ 

 objective was obtained of professedly 140 air angle, 

 and 8/. icxr. paid for it, while, when it came to be accu- 

 rately measured, it turned out only 108, and such a one 

 could have been bought for 50^. It should be the duty of 

 the executive of every microscopical society to secure its 

 members against fraud, and the method above stated 

 would most assuredly do this. 



