168 PROCEEDINGS OF THE SOCIETY. 



simply impossible unless by reducing them to a common medium, 

 ■u'hich (as in the ' balsam angle ' of a dry objective) has no actual 

 connection with the intended action of the lens. The definition 

 of a reveals at once the unequal equivalent of equal angles in different 

 media. The aperture indicated by an angle of say 120^ in a medium 

 like balsam or crown glass exceeds the ajierture indicated by the same 

 angle in air in the exact ratio of the refractive indices 1 • 5 and 1 • 0. 

 The greater value of a (in the formula n sin iv = a) which appertains 

 to the same angular aperture in a more highly refractive medium 

 indicates numerically the increase in the effective rays which is 

 secured for the delineation of the image. Thus the superiority of 

 immersion objectives in all those functions which depend on aperture 

 is directly manifested by the increased value of n obtained by the 

 immersion method.* 



" Instead of the three objectives being denoted by 60°, 53°, and 48° 

 angular aperture (a descending scale), the numerical expression gives 

 •50, '60, and "62 (an ascending scale), and the comparison of their 

 true relations is at once made. 



" Conclusion. — The want of appreciation of the aperture question 

 in this country grievously retarded our progress in optical matters. 

 Years ago, when Continental workers were revelling in the superior 

 capacities of immersion objectives, English microscopists and opticians 

 persistently refused to countenance them : ' the aj)erture was obviously 

 cut down so much by the mounting in balsam ' ; and again, although 

 the use of oil had been suggested by Amici in 1844 and by Oberhauser 

 in 1845, and re-suggested by Wenham in 1855 and 1870, yet oil- 

 immersion lenses were for all these years (and down to 1878) utterly 

 ignored as without any practical utility, and the principle of homo- 

 geneous immersion was at that time entirely missed through the 

 persistence of the fallacy that the ' aperture was cut down.' 



" I may take this opportunity of removing some misapprehension 

 which has existed as to the resolution of the Council that the contro- 

 versy on the aperture question should be ' closed. 'f It is not meant that 

 the subject is to be considered as tabooed by the Society, but simply that 

 when a question, at fii'st doubtful and little understood, has at length 

 by discussion and experiment been demonstrated, the time has come 

 when it is not possible for a Society to occupy the time of its principal 

 meetings by further discussion of the general question. If, after that, 

 any individual Fellow desires to understand and discuss the grounds 

 for the belief, that obviously can only be done by communications 

 inter se or at some of the less formal meetings. It is for this pm-pose 

 that we meet weekly in the Library, and on these occasions, so far 

 from desiring to stifle discussion on any question, the Secretaries and 

 any other Fellows present are always pleased to give any assistance 

 that they can on any desired subject, whether it relates to the circula- 

 tion of the blood, or the law of refraction, or any more difficult topics. 



* See also Professor Abbe's demonstration as to how all tlie principal functions 

 of microscopical vision (the illuminating power of an objective, the depth of focus, 

 and the resolving power) depend upon the expression of a, — Vol. iii. (1880) p. 26. 



t Sec this Journal, ii. (1879) p. 345. 



