214 
CORRESPONDENCE. 
the editors, if assailed on this point, would be quite equal to the 
occasion. 
But there is one passage (p. 169) so pertinent to the controversies 
of the day, that I must give it in the authors' own words : " Es ist 
voUkommen gleichgiiltig, ob der Oeffnungswinkel eines Mikroskops 
beispielsweise 70 oder nur 68 Grad betrage. Es ist geradezu lacher- 
lich, wie Harting mit Recht bemerkt, wenn man bei starkeren Objec- 
tiven, wie es Manche gethan haben, die Grosse des Oeffnungswinkels 
bis auf Bruchtheile eines Grades angiebt. Und ebenso lacherlich 
und unpraktisch ist es. Objective mit Oeffnungswinkeln bis zu 160"^ 
und dariiber herzustellen, wenn hievon wenigstens 40-50° auf einen 
total unbrauchbaren peripherischen Theil des Systems fallen, wie 
diess bei manehen englischen Systemen wirklich vorkommt." 
A translation of the entire work would be out of the question, 
owing to its great bulk, — 628 pages large octavo. Of these about 358 
belong to the microscope proper, while the rest of the book is devoted 
to an exposition of their own peculiar views on gases, crystals, pro- 
toplasms, cell-formation, plant-life, and what not; all very learned 
and very interesting, but which would have gone much better into a 
separate volume, to be entitled, ' The application of the Microscope to 
things in general.' 
Yours faithfully, 
W. J. HiCKIE. 
Mr. J. Mayall, JUN.'s, Critics; and the "Balsam Aperture 
Question." 
To the Editor of the ' Monthly Microscopical Journal: 
224, Eegent Street, London, September 2, 1875. 
Sir, — Mr. Slack's defence of his apology amounts to this : He 
affirms that with Zeiss's ^th pneumo-lens, angle 68°, and C eye- 
piece, with artificial light, he was able to rival the definition of 
Surirella gemma as seen in Dr. Woodward's photograph produced 
with Powell and Lealand's ^tth. immersion, with sunlight. I am 
content to leave that statement to carry its own conviction. 
" Crito's " attempt to answer my 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?" lacks the sagacity he would affect, and does not "perfectly 
account for the phenomenon." He suggests, the luminous field might 
have been obtained by the immersion lens having a larger angular 
aperture than the pneumo-lens used. The very point of the experi- 
mental proof furnished against Mr. Wenham's position in the " Balsam 
aperture question " by his Reflex Illuminator is, that when the pneumo- 
lens is used on a balsam-mounted object (viz. Moller's Probe-Platte), 
the field rays are totally reflected by the cover-glass, — there are no7ie 
to be " picked up," — the total reflexion at the cover-glass is a barrier 
that excludes the pneumo-lens whatever increase might be given to 
the angle of aperture ; but when the high-angled immersion lens is 
