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III. — Measurement of Immersed Apertures. 



By F. H. Wenham, Yice-President E.M.S. 



The measurement of the apertures of Mr. Tolles' glass was thrust 

 upon me. I had no appeal, nor was there any condition that the 

 result was to be private, for the arrival was proclaimed months 

 before it reached me. Had I been forewarned thus : " If the trial 

 favours our statements, say so ; if the reverse, speak not, or we shall 

 denounce you," then I might have found an excuse for refusal. 

 This allegory has literally come to pass. The irritation and rancour 

 that the trial has occasioned amongst a small clique is curious. I 

 am coarsely accused of acting unfairly toward Mr. Tolles — of 

 having performed a trick, and finally it is insinuated that from some 

 mercenary motive of my own that I have disparaged the glass. 

 jMr. Stodder (the owner) makes some remarks, that I need not 

 dwell upon, for his eulogium being headed " Advertisement " 

 carries its own satire with it. Having now no further occasion to 

 take heed of the idiosyncrasies of these worthy people, to them I 

 make my farewell bow, in satisfaction at concluding the question 

 with Col. Woodward. Respecting his ingenious method of measur- 

 ing the fluid apertures of ordinary objectives I have not the slightest 

 oljjection to make. The plan is an admirable one, and quite accurate. 

 To those who still doubt the loss of aperture on objects immersed in 

 transparent media I recommend a trial with a piece of ground glass 

 having a square polished edge. Let the cone of Kght glance over 

 the outer surface, and you have a picture of the air angle. Now 

 tilt up the plate a little, so that the cone of rays enters the glass, 

 and the diminished angle is at once seen and demonstrated on the 

 same spot. 



Col. Woodward's measurements of immersed apertures, falling, 

 as he states, within the hmits laid down by me, removes a question 

 of difierence, and might end the discussion in that respect, but there 

 are one or two sentences that call for brief remark. He says that 

 when the combination was closed as far as possible he failed to get 

 definite results, either in air, water, or balsam. Oftentimes I have 

 found this to be the case, and my custom is (and will be) to set the 

 objective in adjustment on a standard test of known average thick- 

 ness of cover. I was not, therefore, conscious of "escaping any 

 difficulty " by a stretch of liberty, as the first operation was to find 

 the p'oper point of adjustment of the glass, which was tried on a 

 variety of known tests requhiug aperture for their development. 

 The glass was found to be " sluggish," that is, the effect of adjust- 

 ment was not at once apparent. A known Podura, having a medium 

 cover ('005) by which a great number of object- glasses had been 

 set to the mark " covered," was used as the point from which the 



