298 General Notes. [March 
plate, G, resting on the étui, serves as a drawing surface. All 
' parts of the apraratus can be packed in an étui measuring 38 cm. 
X 22.5 0m. X 9.5 cm. o 
The magnifying power varies according to the relative positions 
given to the object-table, objective, and camera. e determina- 
tion of these positions for different magnifications should be 
made before using the instrument. For this purpose a millimetre 
scale may be placed on the object-table, and the camera and ob- 
jective moved until the picture projected on the drawing surface 
has the desired enlargement. The following table, showing the 
positions for given magnifications, was prepared by His. The 
numbers will vary somewhat for different eyes, hence the necessity 
of preparing one’s own table. : 
Upper Upper Diameter 
edge of edge of of field of 
Object- Camera- of vision. 
Carrier. carrier. 
Magnification. mm. mm, mm. 
4 235 5 32 
5 216 7 25 
Objective o 8 115 193 20 
10 100 218 15 
15 83 240 II 
f 15 13 65 oe 
| 20 12 88 == 
25 II 112 8.5 
Objective 1 3° hd 133 ay 
40 II 170 — 
50 10 — 
60 10 230 3 
L 70 10 — 
The lower objective is screwed into the upper, and the higher 
objective into the lower side of the carrier. Fora magnification 
Ryder’s Automatic Microtome.—This new instrument has 
been devised by Professor John A. Ryder, of the Biological De- 
