CORRESPONDENCE. 
155 
So mucli for Mr. Mayall's notions of tlie magnifying power of deep 
objectives. 
Proceeding onward, in the same letter, Mr. Mayall naively remarks, 
that if Mr. Wenliam will try an experiment " on Mdller's Prohe-Platte 
with his Reflex Illuminator and a high-angled immersion lens, he will 
see a luminous field ; whereas, with a pneumo-lens he obtains a dark 
field " ; he then asks the following question : " Whence comes the 
luminous field in the immersion lens if not from its having the power 
to collect rays which are totally reflected when the pneumo-lens is 
used?" 
If Mr. Wenham replied to the question put by Mr. Mayall, he 
might very properly inquire what on earth " Moller's Probe-Platte " 
had to do with the questioUj and whether a plain glass slip would not 
answer the same purpose. The answer to Mr. Mayall's question 
" Whence comes the luminous field ? " is obviously that there is nothing 
whatever in his letter to show that any rays totally reflected when the 
dry lens was used, were picked up by the immersion ; for anything 
that appears to the contrary, the dry had a smaller angle than the 
immersion lens, which would perfectly account for the phenomenon. 
Following this amusing letter by Mr. John Mayall, jun., comes 
one on the same subject from Mr. Jabez Hogg, in which we find a 
most charming proposition. After quoting from Parkinson, who says 
a compound object-glass can be constructed " which shall be at the 
same time both aplanatic and achromatic," Mr. Hogg says : " An object- 
glass may therefore be achromatic and not aplanatic," and hence he 
concludes that some object-glasses which are not achromatic must be 
aplanatic. 
Mr. Hogg thinks Mr. Slack's theory is not coherent. 
Your obedient servant, 
Crito. 
Angle of Aperture of Object-glasses. 
To the Editor of the '■Monthly Microscopical Journal.' 
London, August 10, 1875. 
Sir, — I have no intention of entering into the present object-glass 
question as an advocate of any particular opticians. This I have 
always avoided, even in the case of those who are guided by my advice 
in optical matters. If a partisan comes forward to publish the merits 
of an object-glass of one maker, an opponent is sure to appear in 
favour of another, and the consequence is that a scientific question 
becomes a party one, and degenerates into a mere trade squabble. 
In the last ' M. M. J.' we have a verbose letter from Mr. Mayall, 
assuming the calm indifference of a looker-on amused at " the pro- 
digious display of personalities." It may, however, appear to another 
indifferent " looker-on " that his own letter, from its tone, may vie 
