NOTES AND MEMORANDA. 
347 
undisturbed. This is then drojipcd on to a slide wet with turpentine, 
and very carefully mounted in soft balsam. If slightly crushed in 
mounting, or containing a trace of carbon at some point, the value of 
the object is often increased. 
These preparations can be made with great ease and rapidity, and 
show the construction of the parenchyma, veins, epidermis, stomates, 
and hairs, with great beauty and distinctness. 
In this way was prepared a slide of leaf ashes which was recently 
sent through the circuits of the Postal Club, and which excited an 
unexpected amount of interest and correspondence.* 
A New Device for Dark-Field Dlumination . — I have been quite 
interested, lately, in some experiments connected with the subject of 
dark-ground illumination, and have worked out a device which brings 
out new and most interesting results : 
the accompanying diagram will illus- 
trate its working. 
Let A represent a sectional view 
of the tube of an eye-piece ; B, a 
triple achromatic lens of 1-inch focus, 
and 30° aperture ; C, the diaphragm ; 
D, cap of the eye-piece sliding over the 
tube of the achromatic lens ; E, a thin 
brass plate sliding between grooves in 
the top of the cap, having at the point 
F a small hole, of not more than gL of 
an inch in diameter ; H, the cone of 
rays of the achromatic eye-lens, meet- 
ing at I. 
Place the thin plate so that the hole 
F will be at the side, and as near the 
apex of the cone of rays as possible. It 
is quite evident that none of the light 
usually used by the eye will be allowed 
to pass to it, as it will be interrupted 
by the brass plate. By placing the eye 
at the hole F, and looking at a suitable object upon the stage of the 
microscope, a most wonderful sight will be seen. The object will be 
brilliantly shown upon a dark field. I would suggest its trial upon 
the diatom Heliopelta. It is not necessary to explain the principle of 
the device, as it will be quite evident to those familiar with optical 
work. By revolving the eye-piece in the body tube many curious 
changes in the appearance of the object will take place. In using 
oblique light it will be found best to place the hole on the opposite 
side of the cone of rays from the mirror. The arrangement can be 
used with the common eye-piece, but with an inferior result. The 
value of the device can be tested by making a hole in cardboard with 
a small pin, and holding it at its proper place over the eye-piece.f 
* ‘ American Naturalist,’ vol. xii. p. 704. 
t Prof. Win. Ligliton, in ‘American Quarterly ^licroscopical Journal,’ vol. i. 
2 H 2 
