The Microscope 



AND ITS RELATION TO 



Medicine and Science, 



VOL. III. | 



WHOLE NO. 15 j 



Ann Arbor, August, 1883. 



No. 3 



CONTENTS. 



Original Communications. 



The Measurement of Numerical 

 Aperture. By A. Y. Moore 97 



Sensitive Dentine. By A. M. Ross, 



M. D 99 



Laboratory Work. By C. H. Stowell. 100 

 Foreign Bodies on Coated Tongues.. 104 



PAGE. 



Gleanings from the Journal of the 

 Royal Microscopical Society for 



June. By C. H. Stowell 104 



Correspondence 107 



Editorial Department 108 



Selections 111 



Items ,. 126 



Reviews 139 



®xi%innl &mmn\m\ mtimis. 



THE MEASUREMENT OF NUMERICAL APERTURE. 



BY A. Y. MOORE. 



THE amount of light which objectives would receive from their 

 objects was for a long time expressed in degrees, as measured 

 in air. Dry and immersion objectives were regarded alike until the 

 angle of 180 was reached. Objectives were constantly improved, 

 and it was soon demonstrated that immersion objectives could be 

 constructed which would transmit more light than corresponded to 

 the limit of aperture (180 ) for dry objectives. This then gave rise 

 to the term glass or balsam angle, and it was found that in an im- 

 mersion objective the balsam angle of 82° corresponded with the air 

 angle of 180 . Hence an immersion objective having a greater 

 balsam aperture than 82° was said to have an aperture of "plus 

 180 ," which, of course, is not possible, as an aperture of 180 in 

 any medium — air or balsam — is impossible. When 180° is spoken 

 of, it is understood to mean infinitely near 180 . 



Prof. Abbe made known a system for the expression of aper- 



