100 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
purpose, and even hinting at a trial with a small piece of thermometer- 
tube retaining the mercury-column, under the supposition of the correct- 
ness of the argument used by Prof. Lowne, thinking it might be possible 
that the bright, reflecting, flat surface of the mercury within the tube 
would aid the object in view by producing the desired image, I deter- 
mined to test the same. At the time I was too unwell to carry out my 
suggestions, but I did so at the earliest moment, and my object in this 
article is to state the results. 
The only properly constructed cylindrical lens I possessed was of 
too long a focus for the purpose, which was to try and render evident 
some doubtful markings, dots, lines, or areas on the diatom Amphipleura 
pellucida. To construct a short-focus cylindrical lens means more time 
and trouble than I could give, so I cut off a jiiece of a thermometer-tube 
1/2 in. long and from 1/6 to 1/5 in. in diameter, and having sealed in 
the small column of mercury, 1 mounted it centrally in a thin flat piece 
of ebonite, as the first thing to hand. It was let into a slot diametrically, 
cut exactly to fit the tube lengthwise, keeping its surface parallel to the 
surface of the ebonite. The tube was thus held longitudinally at its 
widest diameter, the flat face of the little mercury-column showing above 
and beneath. It was in this extemporized setting fitted on the top of 
the brass tube of a substage condenser without its lenses, but having its 
own rack work, and being capable of rotation in the centering of the 
substage of the Microscope. Here I had a kind of cylindrical lens 
formed round one axis of revolution, the central portion being blocked 
out by the small column of mercury. No time was lost to now test its 
value as a simple substage condenser for use on lined objects, and also 
as it appeared to me useful to test Prof. Lowne’s theory. After duly 
centering the mercury column, I placed on the stage a slide with Pleuro- 
sigma balticum, and by aid of the plane mirror and daylight, using the 
1/5-in. objective and No. 1 eye-piece, I noticed that the bright reflecting 
surface of the mercury in the little tube did not suffice to give by its 
own light, reflected from the back of the front lens, more than a very 
faint image of the diatom ; but the moment the small tube was decentered, 
so as to place the mercmy-column to one side, or just out of focus, I had 
a very beautiful image of the object, and could by rotation of the tube 
round the central axis of illumination easily bring out, separately, either 
the short horizontal lines or the longitudinal ones by alteration of this 
substage adaptation, or both, showing the markings or areas in s(^uares. 
Another objective was tried, as Zeiss E, equal to about one-ninth. Here 
the image was more perfect, only from its larger numerical aperture 
there was less difficulty to separate the striation. The next trial was to 
go over the same ground again, using simply the divergent rays of the 
Slicroscope-lamp, and with the same result. The divergent rays were 
next made parallel by a bull’s-eye condenser, also by a crossed lens 
before reaching the small tube, which rendered this image very bright. 
Having thus far satisfied myself, I next cut a small piece from a 
solid glass rod of about the same diameter. This was mounted more 
carefully, and upon testing its use in the same manner, I was greatly 
surprised at its efficiency when used to illuminate the same object, and 
also other diatoms. The extreme brightness of the images with a 1/12 
water-immersion made by Gundlach, and selected for me years since by 
