89 



festly 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 

 determined, 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 catalogue of the Bausch & Lomb Optical Co. 



Before the time when the numerical aperture 

 was so thoroughly elucidated, it was known that 

 an increase of angular aperture gave higher 

 results, but just why this was so was not appre- 

 ciated. 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 

 had the same effective aperture. 



How to Measure Angular Aperture. In in- 

 struments of the American type in which the axis 

 of the mirror-bar is in the plane of the stage and 

 in which the circular part is graduated, the mat- 

 ter of measuring angular aperture is quite simple. 

 It was first recommended by Mr. Tolles and has 

 been carefully worked out by Dr. George E. 

 Blackham of Dunkirk, N. Y. After the object 

 has been focused upon, incline the body of the 

 microscope to a horizontal position, remove the 



