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IV. — Angular Aperture of Ohjed-glasses. 

 By F. H. Wenham, Vice-President K.M.S. 



If useful facts are brought to light by the aperture question, I pre- 

 sume that it must continue to be one of interest. I appear again 

 thus early, not in a captious spirit of contradiction, but to notice some 

 fallacies in the methods hitherto used for measuring the aperture of 

 both dry and immersion lenses. The conditions carefully arranged 

 would have been worth bringing before the Royal Microscopical 

 Society, had they not been developed by controversy both strong 

 and ii'regular, in which no Society could properly be involved, and 

 consequently the question must be settled in the same way. 



The controversy was begun by myself three years ago ; I then 

 disputed what I considered to be an erroneous theory, appearing 

 in an essay illustrated by large diagrams, to prove an increase of 

 aperture alleged to be obtained by immersion lenses,* and pointed 

 out how rays were taken fi-om wrong positions in impracticable 

 constrictions. The author of the essay referred to had merely 

 carried the rays into imaginary front lenses, and there deserted 

 them, regardless of their ultimate destination to a posterior conju- 

 gate focus at the eye-piece. The subject has been kept up at 

 intervals by correspondents, with whom I have still to deal. It is 

 difficult to do this without exciting some degree of irritation, 

 because the nature of such strictures implies ignorance of the 

 laws of optics — a science which has long been so exactly defined 

 that there should be no error of the passage of a ray through 

 refracting sm'faces. 



According to disposition, so do modes of controversy difier: 

 perhaps my own is an obnoxious one. Let it be compared. Some 

 always write in the first person singular, and reiterate their views 

 as if to command behef, conceding no credit, and ignoring all 

 rephes except to those who favour their assumptions, with an air 

 that says — 



" I am Sir Oracle, and when I ope my mouth let no dog bark ! " 



This scarcely brings forth fruit. I am always glad to exchange 

 notions with practical working men, and to give my own in ordi- 

 nary phraseology. 



Then there is discussion with a snarl, of which this aperture 

 question affords some examples. This is the least productive of 

 any, for its main strength consists merely in picking out contradic- 

 tions and anomalies of phrase ; science is tossed aside, and its cold 

 reasoning avoided because it is not understood, and abuse is mis- 

 taken for keen argument. Satire may appear in discussion, arising 



* 'M. M. J.,' p. 16, vol. iii. 



