ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 303 



Notes on Aperture, Microscopical Vision, and the Value of wide- 

 angled Immersion Objectives.* 



I. The Aperture Theories. — Apertures exceeding 180° angular 

 in air. — The true notation for Aperture. 



1. The two Theories of Aperture. 



2. " Dry " and " Immersion " Objectives. 



3. Definition of " Aperture.'' 



4. Increase of Aperture vjith the increase in the density of the Meditim. 



— Apertures exceeding 180° angular in air. 



5. The Photometrical Test. — Supposed Identity of the Hemispheres in 



different Media. 



6. The " Resolution " Test. 



7. The " Angular Grip." 



8. Numerical Aperture. 



II. Angular-Aperture Fallacies. 



1. The Hemisphere Puzzles. 



(a) The Convex Hemisphere. 



(b) The Concave Hemisphere. 



(c) The Hemisphere as a Condenser. 



2. Illumination Fallacies. 



3. Power of the Plane Surface of a Lens. 



4. The Diagram Fallacy. (^The Stokes immersion and the ShadhoH dry 

 ' Objectives.') 



5. Fallacies in Practical Construction. 



6. " Not Image-forming Pays." 



7. " Only a Question of Nomenclature." 



III. Photometrical Questions connected with Aperture. 



1. Difference of Radiation in the same Medium. 



2. Increase of Radiation in Glass, Oil, 4'C. 



IV. Microscopical Vision and the Delineating Power of 

 Objectives. 



1. The Abbe Theory of Microscopical Vision. 



2. The Delineating Povxr of Objectives and Aperture. 



V. The Value of wide-angled Immersion Objectives. 



I. The Aperture Theories.— Apertures exceeding 180° angular 

 in air. — The true notation for Aperture. 



During the recent discussion on Aperture many of the old 

 fallacies reappeared, with which we propose therefore to deal in the 

 following notes. It should, however, be distinctly understood that 

 we do so from a wholly impersonal point of view. So far from it 

 being our intention to reflect upon those who gave expression to the 

 views referred to, we recognize that they have in fact thereby done 

 a very useful service, as it has enabled explanations to be given 

 which will serve to prevent any such difficulties disturbing the 

 minds of future generations of microscopists. 



* Taken as read 9tli February, 1881. See p. 365. 



Y 2 



