TRANSACTIONS OF THE ROYAL MICROSCOPICAL 
SOCIETY OF LONDON. 
Description of a Diaphragm Eye-piece for the Micro- 
scope. By Henry J. Slack, F.G.S., Hon. Sec. Mic. Soc. 
(Read October 10th, 1866.) 
In viewing small objects by transmitted light, it frequently 
happens that distinct vision is impaired, and the eye inju- 
riously affected by the large size of the surrounding luminous 
field. If we may compare the objects to an engraving, and 
the rest of the luminous field to a white margin, we shall 
find that different objects require a different portion of mar- 
gin for their most pleasant and efficient display. 
With a given intensity of illuminating power, it is evident 
that the quantity of light affecting the eye will depend upon 
the size of the space from which the light proceeds, and it 
will be frequently found that the whole aperture of an eye- 
piece admits far too much light when the intensity is nicely 
proportioned to the requirements of the object. It is there- 
fore desirable to have a ready means of cutting off the super- 
fluous light by reducing the size of the luminous field, and, 
as objects vary, not only in size, but in their relative propor- 
tions of length and breadth, it is further desirable that the 
field should be susceptible of corresponding changes. 
Having these objects in view, the writer requested Mr. 
Ross to adjust four moveable shutters, so that an A eye-piece 
might be susceptible of all the changes in the form and size 
of its field that different objects would require. This has 
been accomplished in the eye-piece now shown to the society. 
The new diaphragm stops are placed immediately over the 
ordinary round stop of the A eye-piece, and they are worked 
by four milled heads conveniently situated on the flange of 
the eye-piece, and slightly projecting beyond it. Each of 
vol. xv. a 
