ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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horizontal, and lies above the eye-piece of the Microscope. The eye 
sees from above through one side face, at the same time, the micro- 
scopic object and the paper on which the drawing is to be made. The 
rays proceeding from the latter fall on the other side face of the small 
prism, are refracted into this, and so reflected on the gold-leaf that 
they reach the eye in the direction of the rays coming from the 
Microscope. 
A new Ocular Diaphragm.* — Prof. Wm. Lighton writes : — In a 
paper read before the American Society of Microscopists at Indianapolis 
in 1878, I described a new dark-field eye-piece which was the result of 
experiments begun in 1863, and which was also described and illustrated 
in the first number of the ‘American Quarterly Microscopical Journal,’ 
published in 1878 by Prof. Romyn Hitchcock. 
There also appeared in the ‘American Monthly Microscopical 
Journal’ for June 1887 a description of an analysing diaphragm for an 
eye- piece, to be used with the polariscope. 
These two pieces of apparatus were to be used above the eye-piece, 
and were designed for a special kind of work. That now to be described 
is also to be used above the ocular, but for work of another sort. Its 
aim is to intensify the image of a certain class of objects, notably the 
Diatomaceae, and its construction is shown in the accompanying figures, 
fig. 16 being a top view, fig. 17 an end view, fig. 18 an inside view, and 
fig. 19 is a sectional side view of the cap. The same letters in the 
different figures always refer to the same parts. 
A, fig. 19, represents the axis of the Microscope; B, the eye-piece. 
G, fig. 16, the top of the eye-piece in which is a groove J. D is a sliding 
diaphragm moving in the groove from right to left and the reverse, by 
means of the screw F and spring I. Fig. 17 shows the manner of fitting 
the diaphragm in the groove. 
To the under side of the diaphragm is fastened a square post H, by 
means of the screw L. This post gives motion to the diaphragm by the 
use of the screw F, and in the opposite direction by the spring I, which 
is supported by studs K. 
It is very important that a proper adjustment of the diaphragm be 
made. 
C, fig. 19, is the image of the mirror brought to a focal point through 
the eye-lens. It is at this point that the knife-edge of the diaphragm 
should be placed ; the field will then have a subdued tint, and the object 
an exceedingly clear definition. Covering the point C with the dia- 
phragm gives a brilliant image of the object on a dark field , and with- 
drawing the diaphragm from all contact with this point, the object will 
appear as ordinarily seen in the Microscope. I have obtained the best 
resolution of diatoms by the use of an achromatic eye-piece. I have 
also used Steinheil’s 1 in. and 1/2 in. lenses as eye-pieces with good 
results. 
The field under these eye-pieces assumes a soft grey tint the instant 
the diaphragm touches the point C. I do not find the use of the 
Huyghenian eye-piece to be satisfactory. I would strongly advise the 
use of the Nelson ocular. 
Microscope, x. (1890) pp. 8-10. 
