420 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
direction at right angles, the movement of rotation by which the con- 
denser in the older instruments could be drawn out from beneath the 
stage is utilized. The only addition is a screw B which works against 
a fixed projecting piece beneath the stage. This movement is, of course, 
not rectilinear, but on a circumference of 5 cm. radius an arc of 1 to 
2 mm. coincides practically with its tangent. The vertical movement 
of the condenser with its iris-diaphragm is effected by a lever L, which 
acts on a carrier supporting the entire piece. The author considers the 
usual slow movement by micrometer-screw to be an absolutely super- 
fluous complication. 
The iris-diaphragm (fig. 50), which is fixed beneath the condenser at 
an invariable distance from the lower lens, is mounted like the condenser, 
except that the central position is determined by a catch. This central 
position, however, can be passed in either direction so as to obtain the 
effects of oblique light. A circular movement allows the obliquity to 
be directed in all azimuths. 
In the adjustment of the instrument the invariable axis of the body- 
tube is taken as the basis from which to start. With this object, the 
Microscope, armed with eye-piece and objective screwed directly on the 
body-tube, is directed upon a cross-wire which is brought into the centre 
of the field by means of the movable stage described below. The con- 
denser is then adjusted by centering either the summit of its luminous 
cone or the aperture of a small diaphragm above its upper lens. The 
iris-diaphragm is similarly adjusted, while it is opened just wide enough 
Fig. 51. 
to give the diameter of the field. The triple nose-piece with its objec- 
tives is then attached, and each arm is separately adjusted by observation 
of the aperture of the small diaphragm above the condenser which is 
brought to the level of the stage. For thi6 purpose the observer, without 
removing the eye from the eye-piece, first works the screw F until the 
aperture is brought on to the axis of the field from front to back, and 
then the screw V until it is brought on to the transverse axis. 
The movable stage (figs. 50 and 51), designed by the author, is 
intended to remedy some of the inconveniences of the ordinary apparatus. 
It possesses three advantages over the ordinary stage: (1) it can be 
