correspondence. 97 



Mr. Wenham's Demonstrations on the Immersion Aperture 



Question. 



To the Editor of the ' Monthly Microscopical Journal.' 



224, Kegent Street, London, January 12, 1876. 



Sir, — When Mr. Wenham asserts that the angular aj)erture of the 

 image-peucil is the same whether the lens be used dry or with immer- 

 sion, he seems to be imaware that this is equivalent to asserting 

 that the aperture is the same whether used on uncovered or covered 

 objects ! 



In his "simple demonstration," the refutation of his position is 

 contained in the item, — That the lens must in both cases— e. g. wet or 

 dry — be accurately focussed on the surface of the cube of glass. Now, 

 the accurate focus cannot be obtained with a lens adjusted for immer- 

 sion unless immersion contact be made ; the lens when so adjusted will 

 not give a sharp image of an uncovered object : so that when he 

 speaks of adjusting a certain ^^gth for immersion and focussing on the 

 surface of the cube of glass before the -water is introduced, he cannot 

 be describing an actual exi)eriment. It is no valid measurement of 

 the aperture of the image-pencil unless the lens be so adjusted as to 

 give true definition of the glass surface in each trial. There is no 

 such thing, properly speaking, as aperture, unless the image of a point 

 is seen as, approximately at least, a point ; and that can only be ascer- 

 tained either by trial or by going through a mass of laborious calcula- 

 tions. 



Having satisfied myself that the j)osition taken by J)v. Woodward 

 and Professor Keith on the immersion aperture question can be sub- 

 stantiated both theoretically and experimentally, I gladly availed 

 myself of Mr. Wenham's invitation to witness a practical demonstra- 

 tion directed by himself. By the terms of his letter, I was at liberty 

 to have the test made with any lens in my possession. Accordingly, 

 as he had already published a report of his measurement of the 

 angular aperture of a ^th immersion objective made by Tolles, of 

 Boston, and as this measurement did not agree with what I liad 

 obtained with the same objective, I thought it would be more interest- 

 ing to have the trial made with it. The owner of the lens, Mr. Frank 

 Crisp, with great courtesy placed it in my hands for the purpose, 

 together with the semi-cylinder of glass that was described by Mr. 

 Wenham. 



In the 'M. M. J.,' No. Ixiii., p. 112, Mr. Wenham gave a descrip- 

 tion of the method he emj)loyed ; and on p. 116 he wrote that with 

 this identical lens and semi-cylinder, " the balsam aperture with 

 closed lenses was only 68^, . . . and this angle of 68° was the same 

 whether water was introduced or not between the plane surface of 

 front lens and semi-cylinder, taking care to focus (?) in either case " 1 

 He mentions that " a minute stop of leaf metal was placed in the 

 centre [of the plane surface of the semi-cylinder] so as to cut off ex- 

 traneous rays." On the next page he says that he had " rather over 

 than under estimated the aperture from using a stop too large ; less 

 than s-^ih. of an inch would have been more proper." 



