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COEEESPONDENCE. 



Measurement of Immersed Apertures. 



To the Editor of the ' Monthly Microscopical Journal.'' 



Padxal Hall, Chadwell Heath, Essex, 3rd May, 1873. 



Sie, — I am willing to continue discussion so long as any true facts 

 can be elicited from it, that may add to our stock of practical infor- 

 mation. I have stated that with Mr. Tolles must be considered as 

 ended, as my reasoning and experiments seem to be in a style utterly 

 beyond his comprehension. I now make a few remarks on his method 

 of measuring balsam apertures with immersion objectives, as I could 

 only slightly allude to this in my last communications, — having then 

 no precise knowledge of the means by which he obtained such won- 

 derful results. 



In the last 'Journal' for May (Plate XV., page 210) we have a plan 

 of the arrangement. From this, it appears that Mr. Tolles cannot 

 get beyond the idea of putting hemispherical or semi-cylindrical 

 things of glass in front of the objective, so that the light may emerge 

 " without sensible refraction " ? It thus appears that the question 

 stands hopelessly back at the very commencement. It began with this 

 plan, and the causes of fallacy that I then figured and described, will 

 serve still, and need not be repeated. Let us suppose that aperture is 

 to be tested by this wretched adaptation (containing the seeds of a 

 crop of refractive errors), strictly with the intention that the object-glass 

 is in a fair adjustment for giving a correct defining aperture, as fixed 

 by a known object, and not improperly set back so as to place the 

 lenses as close as they will go. Now, the optical question is, what is 

 the loss of aperture which must inevitably ensue, by partly or entirely 

 destroying the refraction at the front surface. Well, bring up your 

 semi-cylindric or hemispheric affair (which we will assume to be so 

 exceedingly well made, that the centre or radius is on the flat surface), 

 focus on the surface, precisely on a point in the centre of curvature. 

 Now, let in your water — will the rays emerge " without sensible re- 

 fraction " ? By no means ! the focal point has shifted its position, and 

 to get an approximate measurement you must attempt to bring the 

 focus again on to the surface. Further, the arrangement will not 

 adapt to various conditions. The loss in water, per se, and other 

 fluids of different refractive powers, cannot be truly ascertained, and 

 every degree of refraction of fluids will again throw the focus away 

 from the plane. The least alteration in the focus in too willing 

 hands will readily favour the desired result in the way that it is 

 expected to go, or give a remarkable facility for developing that un- 

 conscious self-deception, — so characteristic an element in the minds of 

 mediums and spirit-rappers. Speaking plainly, I am astonished that 

 any one having the slightest pretence to optical knowledge should 

 put this forward as more correct, or in preference to simply immers- 



