ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
511 
adapter.’ By this we secure the advantage of the double movable 
thread, without the additional error of the double movable thread, and 
this, moreover, without losing the convenience of a fixed zero. 
This enables you to span your object at equal distances on either 
side of the optic axis, without disturbing the centricity of the eye-piece. 
A guiding line has been added, because an error might creep in unless 
measurements are made with precisely the same portion of the wires. 
The divisions on the screw head have been made white on a black 
ground, on account of their being easier to read in a darkened room. 
A cap to protect the threads from dust and injury, &c., is provided, as 
the threads are no longer inclosed between the lenses, as in the 
Huyghenian form. 
An iris diaphragm is placed below the threads and as close to them 
as possible. 
In spanning the stage micrometer, it will be found better to take the 
readings from centre to centre of the lines, by doing which you avoid 
the diffraction which is always present at edges. 
The measurement of all objects should be performed under a wide 
angled cone of illumination, so that the diffraction at the edges may 
be minimized as much as possible.” 
Two Headings of Scale 50 fx. 
diff. 
A 
1038 
A 
1038 
0 
B 
35 
B 
36 
+ 1 
C 
37 
O 
36 
- 1 
D 
36 
D 
37 
+ 1 
E 
30 
E 
29 
- 1 
F 
37 
F 
37 
0 
G 
65 
G 
64 
- 1 
H 
32 
H 
32 
0 
I 
29 
I 
30 
+ 1 
1 J 
48 
J 
48 
0 
K 
34 
K 
33 
- 1 
L 
29 
L 
30 
+ 1 
M 
40 
M 
40 
0 
N 
45 
N 
45 
0 
0 
38 
0 
37 
! - 1 
P 
44 
P 
43 
- 1 
Q 
39 
Q 
39 
0 
R 
40 
R 
41 
+ 1 
S 
24 
S 
24 
0 
T 
40 
T 
41 
+ 1 
20 
760 
760 
0 
1038 
1038 
H H altered from 33 and 31 respectively. 
An easily constructed Hot-stage.* — A very simple and convenient 
hot- stage was exhibited by Dr. Robert Reyburn at a recent meeting of 
* Amer. Mon. Micr. Journ., xi. (1890) p. 1 (1 %.), 
