46 Transactions of the Boy at Microscopical Society. 
lenses. This statement I cannot but consider as only partially 
correct ; and I would like to divide immersion objectives according 
to tlieir behaviour with the reflex illuminator into three classes: 
Istly, those which give a dark ground ; 2ndly, those which give a 
grey ground ; and 3rdly, those wide - angled immersion lenses 
which are capable of giving a light ground, and which alone are 
suitable for the method I am about to describe. 
Some time ago, whilst trying Wenham’s reflex illuminator 
and Powell and Lealand s | new formula immersion lens by 
unmodified sunlight on Moller’s Probeplatte, I found on lower- 
ing the condenser that the field became brightly illuminated by a 
spectrum. I focussed for one of the larger diatoms, and found it 
bathed in spectral colours, which impinged on it very obliquely, 
and which brought out the markings very distinctly. Bringing 
successively the finest diatoms of the Platte, such as Navicula 
crassinervia, Nitzschia curvula, and Amphipleura pellucida in the 
field, I found all their cross markings revealed with wonderful dis- 
tinctness, the last-named diatom appearing like a fine comb. 
These cross markings, for which I had been looking for years, and 
which had baffled all my efforts to show them, appeared now all at 
once distinct enough to be counted, and were entirely free from 
spurious and coloured fringes. I then tried the so - called 
Edmund immersion paraboloid in the same way, and found that 
by its means similar but not so satisfactory results could be ob- 
tained. The secret of my success was that I had used castor-oil 
instead of distilled water to unite the top of the reflex illuminator 
with the under side of the slide, and the former being a thicker 
fluid than the latter, allowed me to obtain that range for focussing 
the illuminator which is necessary to obtain light ground. I then 
experimented with lamplight, and found that the more powerful 
the light the more distinct the striae appeared, and that the Bal- 
linger lamp gave the best results ; but that even by the light of a 
lamp having a wick of only half an inch broad, the markings were 
plainly shown. The illumination obtained in this way, unmodified 
sunlight included, has the advantage of not being by any means 
intense enough to be painful to the eyes, and as required, any of 
the colours of the spectrum may be used without the slightest 
difficulty. The blue and violet rays are especially agreeable to the 
eye. The light becomes most dispersed when the narrow side of 
the wick is used, and when the rectangular prism is turned a little 
on its vertical axis so that the rays do not fall quite perpendicu- 
larly on one of its sides. 
Finally, I will give a few practical instructions for this 
method. I place the lamp to the left, and when using the ordinary 
form of Moller s Probeplatte, or a slide having a balsamed valve 
of A. pellucida lying horizontally on it, I turn the reflex illu- 
