168 MEMORANDA. 
too small to be measured with the photometer. I then placed 
two different surfaces together in a straight line, so that half 
a small beam of light (from a candle enclosed in a magic 
lantern, from which the lenses had been removed) was re- 
flected from one surface, and the other half reflected at the same 
angle from the next surface. The light reflected was received 
on a sheet of paper some fect distant. Jn this way, the light 
being feeble and the room dark, the difference of illumination 
could be easily detected by the eye. When one surface was 
dense flint, and the other optical crown glass, the difference, 
at an angle of 45°, was only just appreciable. The quartz 
surface reflected more light than either. I then coated one 
surface of a dense flint prism with diluted albumen, and this 
was compared with the surface of a second prism made of the 
same glass. There was less light reflected from the albumen, 
but the difference was surprisingly small. These experiments 
led me to the conclusion that the working of the surface is 
more important than the quality of the glass; also that coat- 
ing with albumen would not make sufficient difference to 
compensate for any inconveniences that might be caused by 
air-bubbles or irregularities of the film.—W. Hueains.’— 
F. H. Wennam. 
How to make Diatoms Stick.—In your last Journal (April) 
Mr. Ward asks how to make diatoms stick on a slide after 
being arranged. I have done a little in setting diatoms, and 
never had any trouble with their moving when covering. My 
plan is to centre the slide with a spot of ink on the reverse 
side before putting it on the stage, then with a dipping-tube 
to take a drop of the water the diatoms are in, put it on the 
slide, but not on the centre; now with a bristle and the aid 
of an object-glass pick out the diatoms wanted, and push them 
to their position with a little water adhering to them. When 
the number are arranged, dry the slide over a gas flame, and 
mount in the usual way with thin balsam. I find, when the 
diatoms are perfectly dry, that it takes some force to move 
them, and some of them will even break before they can be 
moved.—R. Lricu, Aberdeen. 
Price of Vuleanite Cells, &e.—You will oblige by correcting 
an error in your report of my remarks on vulcanite cells pub- 
lished in the last number of the Journal, p. 112. ‘The price 
