correspondence. 47 



Mr. Mayall's Letter. 

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



Sir, — I have no intention of discussing optical questions with 

 Mr. Mayall. My paper referred to (which he so genteelly denies), 

 demonstrating a means of obtaining full aj^ertiires on immersed 

 objects, was contributed by me to th ' Quarterly Journal of Micro- 

 scopical Science,' No. 12, July 1855, page 302. It did treat only of 

 immersed apertures, under the title " On the Aperture of Object- 

 glasses in relation to Objects in Canada Balsam." 



In expressing his dissent against an article that he has not read, 

 and saying that it has " no reference to the subject," Mr. Mayall 

 commits himself to a mere blind and senseless contradiction. 



The cube of glass with which my " simple demonstration " is 

 tried measures three inches and one-tenth square. If an object-glass, 

 say a yVth, is adjusted for immersion and focussed on to surface, the 

 disk of light observed, and then water introduced, the disk remains 

 the same. If the object-glass is now raised (as it should be) so as 

 again to meet the surface for the immersion focus, it will of course 

 also slightly raise the apex of the cone in the glass ; but still the 

 increase of the angle in the distance of the cube is too small to be 

 detected. 



Mr. Mayall takes the air angle as shown in the glass for un- 

 covered objects, then adjusts the lenses for immersion, and attributes 

 the increase of angle always thus obtained by a nearer approximation 

 of the lenses, to the introduction of water, or the immersion ! ! This 

 needs no comment. 



Professor Stokes would perhaps think it a profitless waste of time 

 to wade through the wearisome length of this controversy. Mr. Mayall 

 imagines that he has secured him as an ally. I need not question 

 the Professor's authority, but I can only say that if he shows mathe- 

 matically that the principles involved in my " simple demonstration " 

 are not right, I shall have no hesitation in demonstrating practically 

 that he is wrong. 



Finally, if Mr. Mayall, or anyone that yet believes that in ordinary 

 immersion object-glasses, the angle in glass is increased by a water 

 connection between the front lens and glass surface, I shall be happy 

 to give a demonstration with any object-glass that he may choose to 

 bring. I am in town every day, with all necessary appliances at 

 hand, and with due notice would have all ready, so that no time 

 should be lost. 



I am. Sir, yours obediently, 



F. H. Wenham. 



VCL. XV. 



