78 A New Illuminating Apparatus for the Microscope. 
by which means the mass of the slide itself, the thin interval be- 
neath it being disregarded, forms the continuation of the upper lens 
of the condenser. When in special cases it becomes a matter of 
importance to avoid a loss of light as much as possible, that interval 
may be filled up by the application of a drop of water. 
The condenser is not made achromatic for the reason that, for 
the efiect contemplated, it would be altogether useless to seek to 
obtain a sharp image of the cloud or other source of light, as it is 
in like manner quite immaterial whether the image is formed pre- 
cisely on a level with the object, or somewhat below or above it. 
On the other hand, attention has been directed to giving the con- 
denser as large an angle of aperture as possible for the superior 
focus. It furnishes — especially with the design of producing lumi- 
nous positive images — even rays which in a film of water are in- 
clined at almost an angle of 60^ to the axis, and which therefore, 
as the limiting angle of total reflexion is about 48°, could never be 
conveyed to the object out of an interval of air. If, however, rays 
of such an inclination have to be had recourse to for any purpose, 
of course the object must not be mounted in air, and the interval 
below the slide must in like manner not be occupied by air, but be 
filled up with water. 
The rays of the primary source of light are conveyed to the 
condenser by a plane mirror B, which does not admit of a lateral 
movement, but turns only upon a fixed point in the axis of the 
instrument. The regulation of the illumination is effected by 
means of a special diaphragm-frame c, a few centimeters below the 
stage of the microscope between the plane mirror and the condenser, 
near the inferior focus of the latter. The effective portions de- 
tached from the entire surface of light available by means of the 
different diaphragms, act, with respect to the object, as though they 
were luminous surfaces very far removed, but of a suitable dimen- 
sion. In order to be able to change the stops rapidly and with 
readiness the diaphragm-frame is made to turn bodily upon a pin 
at the side, c. It is thus rendered easily accessible, and while it 
can be readily turned outwards from under the stage of the micro- 
scope, a single movement of the hand will restore it to its correct 
central position. This frame is destined not only for the direct 
reception of the stops, but also for supporting on it a disk, pro- 
vided with a rotating and a slipping motion. When the entire 
frame, fitted with a diaphragm in its aperture, has been brought 
into position below the stage of the microscope, it is by means 
of the motion given to the disk that all the modifications in the 
direction of the incident rays are brought about. A rotating and 
a sliding motion are given simultaneously by a projecting handle 
with a milled head, e, below the stage of the instrument. The 
action of this handle, when turned on its axis between the fingers, 
