ABBE, AND MICROSCOPE THEORY IX GERMANY. 163 



Pseudoscopic Effects in the Binocular Microscope"; XVI., "On 

 the Estimation of Aperture in the Microscope " ; XVII., " The 

 Relation of Aperture and Power in the Microscope " ; XVIII., 

 " On the Mode of Vision with Objectives of Wide Aperture " ; 

 XIX., " Xote on the Proper Definition of the Amplifying Power 

 of a Lens or a Lens-system " ; and XXL, " On the Effect of 

 Illumination by means of Wide-angled Cones of Light.'' 



Among English microscopists the knowledge of Abbe's works 

 was more general than on the Continent ; and although a certain 

 number of discordant voices were raised, the principal theorems 

 of the new theories were intelligently appreciated, particularly 

 among amateur microscopists. 



If we leave aside the ridiculous dispute over the aperture 

 question, it must be gratefully acknowledged that Abbe's 

 numerous enlightening statements in the Journal of the Royal 

 Jfic?'oscopical Society were occasioned precisely on account of these 

 discussions in English microscopic literature. In this way the 

 confusion concerning stereoscopic effect was solved, first by the 

 construction of stereoscopic oculars (Treatise XIII.) and then by 

 the lucid statement of the conditions which govern the origin 

 of orthoscopic and pseudoscopic impressions. 



The treatise on the measurement of the aperture contains 

 more rigorous statements as to the sine-condition and explana- 

 tions concerning the influence of aperture on the clearness of 

 images and the imaging power of the microscope. We also find 

 here important observations regarding the images of isolated 

 corpuscles or flagellae, whose diameters are fractions of a wave- 

 length of light. Referring to this, Abbe says (p. 362) : " Such 

 objects can be seen however small they may be. This is only a 

 question of contrast in the light effect, of good definition of the 

 objective, and of sensitiveness of the retina." And in a note he 

 expressly points out, to avoid all possibility of misunderstanding, 

 that neither Helmholtz nor he himself had at any time spoken 

 of a limit of visibility, but always of the limit of " visible 

 separation." It is well known how both these things have often 

 been confused, even quite recently, since the apparatus con- 

 structed by Siedentopf and Zsigmondy, for the rendering visible 

 of ultra-microscopic particles, made it possible, by appropriate use 

 of dark ground illumination, to see particles the diameters of 

 which were only a few millionths of a millimetre. 



