CORRESPONDENCE. 
95 
time say that, taking his series from ^th up to ^^th. and testing them 
against similar objectives by Powell and Lealand and by Hartnack 
and Prazmowski, on the whole of those diatoms known as test-objects, 
■ — on Podura, on Gnat's body-scale, on Dr. Carpenter's " thread-cells," on 
various specimens of tissue, on the double-star test, on Noberfs Test- 
plate, by the test of deep oculars, and with all the methods of illumi- 
nation that are in vogue, — the English and French objectives carried 
the palm. I do not here make allowance for any advantages there 
may be in the ease of manipulation in consequence of Zeiss's objec- 
tives being of smaller angular aperture and having greater working 
distance ; my judgment is upon the results obtained, which I under- 
stand to be of the first importance. The mere item of one objective 
being a little less difficult to manipulate than another does not appear 
to me to weigh against superior clearness and definition in the imago 
obtained. 
And now, what, as to the latest developments in microscopy dis- 
cussed by the Hon. Secretary ? When Professor Abbe is eulogized for 
having " minimized the angles of aperture," it appears to me he is 
receiving praise for a backward march. In high powers — all those 
beyond ^th — what we seek is magnification with clearness and defini- 
tion. This is obtained if a lens be made that will support deep ocu- 
lars. It is well known that the power of a lens for supporting deep 
oculars is in proportion to the perfection with which the corrections 
are made throughout a given angular aperture ; if these corrections 
extend up to the maximum of possible aperture, such a lens bears the 
deepest oculars before the optical image is broken up. 
Professor Abbe is largely quoted, as though he were an impartial 
authority criticising Zeiss's work ; whereas, he is simply saying what 
he can as an advocate for the work which Zeiss is producing under 
his direction. When he says that " corrections cannot be well made 
with dry lenses exceeding 105° to 111° aperture, without a consider- 
able reduction of working distance," he is simply stating a fact ; but 
this fact does not presume that Zeiss's F lens ( = yV*^) ^^^^ aperture 
of 105° can for a moment hold its own on the highest tests against 
Dallmeyer's or Powell and Lealand's -|^ths with apertures beyond" 
140° : the working distance is different, and so is the quality of the 
optical image. If Professor Abbe is satisfied with his result : so be it. 
But when I and my most critical friends have made the comparison, 
we first of all note that a deep ocular quickly finds the limit of light in 
Zeiss's lens, that it breaks up the crispness of image, that, in fact, the 
lens has the qualities of similar low-angled lenses such as were pro- 
duced in England many years ago. 
Again, according to Mr. Slack, the most recent investigations and 
experiments — those made by Professor Abbe — have led to the con- 
clusion " that even in immersion systems, for the normal requirements 
of science, there would be no loss, but in many respects a gain, if they 
were constructed with smaller angles of aperture." I am unable to 
determine what Professor Abbe means by the normal requirements of 
science, but certainly one of the requirements is to render visible what 
was before invisible in the object : how this is attained by diminishing 
