100 
CORRESPONDENCE. 
I am sure I am correct in stating that tlie generally received 
opinion regarding glasses of very high angle of aperture is, that they 
are "optical curiosities of no practical use " (Dr. Pigott) ; " their use 
limited to particular classes of objects" (Dr. Carpenter); in short, 
that a glass of nearly 180° is per se good for nothing but to show 
dif&cult test-objects under extreme obliquity of illumination, and 
comparatively valueless for the study, under central light, of olDjects 
coming properly within the sphere of the histologist and " working 
microscopist." My friend, G. E. Smith, referred our Society to Mr. 
Tolles' new " four-system " objectives as a practical refutation of this 
doctrine, and challenged us to the proof. 
In my previous note I gave no names of makers of the various 
objectives, lest I might appear desirous of " puffing " the work of some 
favourite optician at the expense of others. Since this is considered 
an error, I will now say that the glasses tried were by Tolles, Wales, 
Zentmeyer, Smith and Beck, Powell and Lealand, Nachet, Scheik, 
and Siebert. All were bought for first-class specimens of the various 
makers' skill. As our object was not to make a qualitative analysis of 
the general merits of the various glasses, we confined ourselves to such 
objects as properly come under the notice of the histologist, and used 
only central illumination. After as full and fair examination as we 
were able to make, we unanimously agreed that the new "four-system" 
form of construction, as invented by Mr. Tolles, is an actual proof 
that objectives of the highest attainable angle of aperture can be so 
constructed as to meet the narrow-angled glasses on their own ground, 
and not only equal but actually excel them. When I state that 
certain glasses "broke down" under deep eye-piece, I meant that 
the difference in favour of the wide-angled glass under these con- 
ditions was so striking as to leave no room for comparison. In short, 
this glass — Tolles' new " four-system " ^V^h of 180° — so far from 
being a mere " costly toy " of " no practical use," will, if properly 
handled, give better results in the hands of the " working micro- 
scopist," on any object suited to its magnifying power, than any of the 
best glasses of moderate angle of aperture which I have yet examined. 
In naming the angle 180°, I wish to be understood as simply giving it 
as named by the maker. 
I feel sure that the best and most thoroughly scientific " working 
microscopists " in this country no longer discard an objective simply 
because of high angle, as being therefore unfitted for the best work. 
On the contrary, the narrow-angle is used for "blocking out" the 
work, as it were ; the wide-angle for finishing it off. Whether their 
work is " valueless " or not, let the pages of this valuable Journal 
testify. 
In conclusion, I wish to state that I in no sense submit this note 
as an official expression of the views of our committee on this subject ; 
it is to be taken as a hasty expression of my own opinions alone. 
Yours faithfully, 
Albert F. Dod. 
