252 
NOTES AND MEMORANDA. 
a diaphragm located in the focus of the equivalents of both lenses. 
The field of the new eye-piece is considerably larger and flatter than 
that of Kellner’s, and the image is sharply defined to the extreme 
edge.” 
NOTES AND MEMORANDA. 
Death of a Diatomist. — The ‘ Journal of Botany ’ (August) states 
that the death is announced of Dr. Gustav Waldemar Focke at 
Bremen, on June 1, in his sixty-eighth year. He was well known 
for his researches in Desmidece and diatoms. 
Dry-mounting of Crystals. — Mr. R. S. Peet, in the ‘ Cincinnati 
Medical Journal’ for August, says: — “My attention was particularly 
called to crystals, separate from plants, by a discovery made by me 
last December. In experimenting for the best oblique illumination, 
I was one night startled by finding the field (of diatoms) suddenly 
filled with gems such as only polarized light can yield, while at the 
same time the ground was perfectly black. On looking at the mirror, 
I saw that I had unconsciously carried it quite to the left (the lamp 
side) of the axis of the objective. Noting the position of things, I 
was the following night able to reproduce the effect. I diligently 
followed the thing up, until I reached what I will now describe : I 
place the kerosene lamp without a screen, about fifteen inches to the 
left of the stand, the latter being at right angles to the former. A 
slide of Nottingham earth is placed on the stage, and focussed by 
direct light. The mirror is then slowly carried to the left until the 
field is nearly dark. Afterwards the mirror is very carefully moved 
away from or toward the observer, as it were feeling with the utmost 
minuteness for the angle. When the angle is hit, it will be manifest 
by the diatoms appearing illuminated with polarized light, those best 
formed for polarization showing as the most exquisite gems conceiv- 
able — indeed passing conception. That the light is really polarized 
is, I think, proved by the fact that by a slight shifting of the mirror 
the complementary colours are shown in the same object. By a hori- 
zontal arrangement I have adapted my bull’s-eye to the use of a con- 
denser. By careful adjustment I get rid of all decomposed light, 
obtaining a perfectly achromatic result, and very greatly increasing 
the intensity of light. When the right angle is secured without the 
condenser, the latter is placed close under the diaphragm, concave 
side up. The faint spot of light on the cover is made to present 
itself just to the right of the objective. I then adjust with my eye 
upon the tube, if the effect is not reached, the condensed light is 
caused to come directly under the objective, the mirror being after- 
wards carried a little farther to the left, and adjusted as before. 
Diatoms and polycystines are the objects most easily exhibited, then 
