CORRESPONDENCE. 
215 
used, field rays enter tlie lens, giving a luminous field — rays from 
balsam of greater angle tlian 41° — the very rays “ challenged ” by 
Mr. Wenham. The optical law that forbids the pneumo-lens to 
gather the field rays from the Illuminator also excludes it from all 
rays nascent from the object of greater obliquity than 41° ; and con- 
versely the law that has permitted the field to be luminous in tho 
immersion lens with rays beyond 41° permits image-forming rays to 
pass from the object beyond 41°. Whether the rays pass direct from 
the total reflecting surface of the prism, or from the object, if they 
reach the posterior surface of the front lens at equal angles of inci- 
dence, they both follow the same process of refraction. That image- 
forming rays deflected by the object, or nascent therefrom, of greater 
angle than 41° do enter the immersion lens and are refracted into 
the optical image is, I conceive, abundantly proved, — theoretically 
by Professor Keith’s computation, experimentally by the fact that 
definition entirely invisible to the pneumo-lens and manifestly the 
product of earfra-oblique rays is visible with the immersion. Those 
who care to follow the subject will at once note that the increase 
of angular aperture due to the immersion principle is not the increase 
“ Crito ” speaks of. 
The answer to my question is, as Dr. Woodward and Professor 
Keith demonstrated : That immersion lenses made on certain formulae 
transmit a greater angle of rays than corresponds to the maximum air-angle. 
I take this opportunity of saying that shortly after the arrival of 
Dr. Woodward’s photographs of Professor Keith’s computation, I 
drew rip a brief history of the “ Balsam aperture question,” and sub- 
mitted it with the computation to one of the highest mathematical 
authorities in England : the result was against Mr. Wenham. I should 
have made this known, but I understood Mr. Wenham to reject the 
experimental proof as being inadmissible in deciding the question of 
image-bearing aperture. It remained to provide means by which rays 
beyond 41° from balsam should be refracted through the immersion 
lens ; and this was required to be done with legitimate means— such 
as admitted of no cavil on the score of being made only for the 
purpose of proof and of no moment in practical microscopy. The 
Keflex Illuminator is surely not open to this kind of objection ? 
I am, &c., 
' John Mayall, jun, 
Mr. Garner’s Bucephalus. 
To the Editor of the ‘Monthly Microscopical Journal.' 
3, Queen Street Place, Upier Thames Street, 
London, September 7, 1875. 
Sir, — Having sent a copy of my letter to you of July 30th date 
to Mr. Garner, he has in return kindly lent me the paper published 
in the ‘ Zoological Transactions,’ December 8, 1835, to which he 
referred in his communication to your Journal of July 11, and 
accompanied the same with the following note to me, viz : 
“ Dear Sir, — I send you my paper in which I figure a parasite which 
