94 
CORRESPONDENCE . 
the next sentence he says it was “ drowned in light ” !) With sundry 
other objectives, such as Zeiss’s C and D ( = ^th and ^th), he also 
obtains admirable definition of this diatom. Further, that, taking a 
delicate valve of P. angulatum that was not satisfactorily exhibited by 
Zeiss’s C and D objectives with illumination from sub-stage mirror or 
condenser, he has instantly increased the resolving power by the em- 
ployment of the Reflex Illuminator. Later on he tells us he has had 
an opportunity of trying experiments with Powell and Lealand’s new 
-gdli, and he says the view he obtained of P. angulatum, with an ocular 
giving a magnification of about 2000 linear, “ surpasses in beauty and 
brilliancy anything seen before.” For all this information we are 
thankful ; if it be rather vague and fragmentary, it comes from good 
authority, and doubtless some of your readers will receive it with that 
polite deference which such authority seems to require. 
From the limited nature of the severity of the tests he mentions, 
viz P. hippocampus (which Moller has thought to be too vulgar a 
test to be admitted in his series on the Probe-Platte), and P. angula- 
tum (which Hartnack exhibits with plane mirror axial illumination 
with every ifh objective that leaves his hands), I am inclined to 
think that Mr. Slack has reached that stage in microscopy when the 
ardour of research palls, and we rest satisfied with mere amplification 
of an easily obtained image, rather than search for an image that taxes 
all our skill in manipulation and all the defining power of the finest 
lenses at our command ; otherwise I should have thought he would 
have selected some such admittedly difficult test as the Gnat’s body- 
scale, in which there is structure that challenges the power of the finest 
lenses to exhibit. It is clear, however, that Mr. Slack cared little for 
settling knotty questions ; it was far easier for him to take the humbler 
position of a reviewer of old — old tests, the definition of which is pretty 
generally admitted : and this he has done. It is strange that he 
learned only within the last year “ the amount of caution required ” to 
regulate the quantity of light to be used with small and with large- 
angled objectives. Even now he seems not to have mastered that 
subject, for he speaks of a certain “ J-th with less aperture ” showing 
P. hippocampus “ with same eye-piece rather better ” than Powell and 
Lealand’s -gth “because not so much drowned in light.” Surely, a very 
important part of the process of manipulation in microscopy consists 
in duly proportioning the light to the capabilities of the lens ? If an 
image appears to be drowned in light, it shoidd be the duty of the 
microscopist to rescue it from that unfortunate j>redicament. He 
favoured Zeiss’s C lens on the same object by carefully screening off a 
little superfluous light “ by holding a sheet of white paper, so as to 
stop some rays from entering the field ; ” and he makes suggestions 
about placing the microscope “ some way from the window, where there 
is a little shade,” and about surrounding the eye-piece “ with a screen 
of black cotton velvet, to keep all glare from the eye,” with an air of 
naive earnestness that is quite interesting. 
I, too, have had an opportunity of examining some of Zeiss’s ob- 
jectives — about twenty of them — and, while making the general admis- 
sion of their excellence and evenness of quality, I must at the same 
