optical capacity. It is manifestly difficult for opticians 

 who produce large quantities of objectives to measure 

 and suitably mark each individual product, particularly 

 when the differences are at best slight. 



It is easy to learn the numerical aperture of an 

 objective after the angular aperture has been deter- 

 mined, as the various values for different angles have 

 been computed and are issued in tables. Such tables 

 can be found in the pages of the Journal of the Royal 

 Microscopical Society and in the latest edition of 

 Carpenter's " The Microscope and its Revelations," 



Before the numerical aperture was accepted as the 

 absolute measure of the optical efficiency of an objec- 

 tive, it was known that an increase of angular aperture 

 gave better results, but just why this was so, was not 

 appreciated. So also was it difficult to understand that 

 a dry objective of 180 degrees, a water immersion of 

 96 degrees, and an oil immersion of 82 degrees angular 

 aperture had the same optical efficiency. 



How to Measure Angular Aperture. With 

 instruments in which the axis of the mirror-bar is in the 

 plane of the stage and in which the circular part is 

 graduated, the matter of measuring angular aperture is 

 quite simple. It was recommended by Mr. Tolles, and 

 has been carefully worked out by Dr. George E. 

 Blackham. After the object has been focused upon, 

 incline the body of the microscope to a horizontal 



80 



