243 
IX.— A LOW-POWER EYE-PIECE WITH LARGE FIELD. 
By H. Hartridge, M.D., Sc.D., F.R.M.S., Fellow of 
King’s College, Cambridge. 
{Read March 15, 1922.) 
One Text-Figuke. 
One of the disadvantages of the Huyghens type eye-piece is that 
the lower powers possess so small a field of view compared with 
those of the high powers. The eye-pieces of different magnifying 
power in a series should have the same size of field, whereas 
actually the size is found to decrease as the magnifying power 
decreases, as the following Table shows : — 
Ordinary Eye-pieces. 
Magnifying power . . 
X 5 
x 6 
X 8 
X 10 
X 20 
X 25 
Size of field in cm. as seen 
at 25 cm. from the eye 
11-7 
13 
14*3 
16-8 
24 
33 
Zeiss Compensating Eye-piece. 
Magnifying power . . 
X 2 
X 4 
X 6 
X 8 
X 12 
X 18 
Size of field in cm. as seen 
at 25 cm. from the eye 
7*6 
12*7 
14-3* 
12*3* 
12*6 
19 
* The break between these two readings is due to the change from Huyghenian 
to Kamsden type. 
The restriction of the field of the low-power • lenses is partially 
due to the limit set to the diameter of the lenses of the eye-piece 
by the internal diameter of the draw tube of the microscope, and 
partially due to the design, for in all the eye-pieces at present 
available (i.e Ramsden, Kellner, and Huyghenian) the illuminated 
area bounded by the eye-piece diaphragm must be considerably 
smaller than that of the largest lens found in the eye-piece. 
For a larger field to be obtained, either the diameter of the 
draw tube must be increased or a different design of eye-piece 
employed. Such an eye-piece can be constructed with the 
approximate lens positions shown in the diagram. 
All the lenses are ground from common crown glass. The 
lower lens or collimator is placed at such a distance from the 
objective that the back lens of the latter is at the former’s lower 
focal plane ; it therefore receives the divergent bundles of rays 
from the objective and renders them parallel. (The individual 
