CORRESPONDENCE. 
301 
greater than 82° of ' image-forming rays,' unless he can show some 
material error in Mr. Keith's computations." 
In replying,* Mr. Wenham discovered no better ground on which 
to base his objections than to suggest that the data given by Mr. ToUes 
were uncertain. He said, " Mr. Tolles, from whom all measurements 
come, has repeatedly supplied diagrams to suit his purpose ; " and 
again, " Of course Messrs. Woodward and Keith are not responsible 
for data, but the former gentleman tells us it is ' a diagram accurately 
constructed in accordance with the computed results ' ; having therefore 
been drawn to suit the proposition, it may be dismissed " ! This 
estimate of the value of his opponent's diagram differed from that he 
gave of his own with his " New formula," thus :] " Diagrams, how- 
ever, are surprisingly accurate in their capability of indicating causes 
and results in the microscope and object-glass." 
For my part, the question being put in the form of a mathematical 
demonstration, I considered no authority would be recognized as of 
value in criticising it except that of a professional mathematician ; I 
therefore placed it in competent hands. Mr. Wenham says that 
" however consoling to Mr. Mayall the opinion of the high mathe- 
matical authority may be, his anonymous verdict will not be considered 
important by others." Let me assure Mr. Wenham the verdict is none 
the less consoling to me, nor will it be considered less important by 
Dr. Woodward, Professor Keith, and others, from the fact that it was 
given by Professor G. G. Stokes, Secretary of the Eoyal Society. 
Your obedient servant, 
John Mayall, jun. 
The Aperture Question.;]: 
To the Editor of the ' Monthly Microscopical Journal^ 
Sir, — Nothing can be easier than to dispose of Mr. John Mayall, 
jun.'s, claim to have discovered a " most elegant practical " refutation 
of Mr. Wenham's position on the aperture question. 
His claim is based on the supposition that in the Eeflex Hlumi- 
nator none of the rays are transmitted when a dry objective is used 
on " Moller's Probe-Platte " (!) ; or, in other words, that all tlie rays 
fall within the critical angle, and are consequently totally reflected 
from the upper internal surface of the slide. 
If your readers will refer to the seventh volume of the ' Monthly 
Microscopical Journal,' page 241, they will find in Mr. Wenham's 
own paper, when describing his new appliance, the most complete and 
conclusive refutation of this assumption; for after saying that he 
will anticipate a few objections to the arrangement, he adds : 
" It may be said that the rays reflected from the lower end of 
the facet are just without the angle of total reflexion, and might 
enter true, and I had intended to stop off a small segment of the lens 
at this place, but found it so desirable, in many objects, to admit a little 
light, that I preferred it without alteration. It is easy to get a black 
field in all cases by mere mirror adjustment." 
* ' M. M. J.,' No. Ixxi., p. 221. t Ibid., No. lii., p. 164. 
[X This letter has stood over from the last number. — Ed. ' M. M. J.'] 
Y 2 
