142 MICROSCOPIC VISION. 



Microscopical Club. By these publications, then, all 

 English - speaking microscopists were made acquainted 

 with what is now known as " The Abbe Theory/' 



We must now pause to investigate what were the ideas 

 concerning the theory of the microscope current at the 

 time the diffraction theory was published. In the first 

 place, Dr. Goring had in 1837 shown by experiments per- 

 formed on lepidopterous insects' scales, with an early 

 achromatic ^ objective of 27|° of aperture (an uncemented 

 triple, consisting of two biconvex and one biconcave lenses), 

 that resolution was connected with angular aperture. 

 *' The gi^eater the angular aperture the more could be 

 seen," was twenty years ago held to be an axiom. The 

 effect of this was to produce keen competition between 

 the opticians in the production of objectives of the widest 

 angle. This naturally led to the measurement of angular 

 apertures, and out of the measurement of the angle rose 

 the great aperture controversy. Now Prof. Abbe's theory 

 has quite disposed of this question of angular aperture by 

 showing that resolution varies as the sine of the semi- 

 angle and not as the angle itself. Thus in the case of 

 objectives of 54° and 130° of angular aperture an increase 

 of 100% in resolution is obtained by an increase of no less 

 than 140% in angle ; but the increase of 9|% in an angle 

 of 160*^ to 175° is accompanied with less than 1J% of in- 

 crease in resolving power. It was the popularizing of 

 facts like these derived from the Abbe theory that put 

 an end to 'Hhe battle of the objectives." No one now-a-days 

 would think of paying £10 to £15 for an extra 5° in a 

 wide-angled objective. Further, the exposure of the fallacy 

 of the utility of objectives possessing enormous initial 

 magnifying power is also due to the Abbe theory, conse- 

 quently 2-V, -3V, 5V3 "eV) "sV of limited apertures are extinct 



