80 A New Illuminating Ap;parakis for the Microscope. 
glass — the apparatus does not quite come up to the performance of 
a concave mirror. The reason thereof hes in part in this, that the 
rays being considerably inchned suffer a perceptible diminution of 
their intensity ; it is partly due to the circumstance of the unavoid- 
able reflexions at the surfaces of the condenser, when the aperture 
of the diaphragm occupies an excentric position, producing secondary 
images underneath the object, virhich by the share they take in 
the formation of the image cannot but impair the effect. This 
defect could probably be got over only by means too complicated to 
be of practical use. But since with central illumination this defect 
is entirely absent (for in that case the images arising from reflexions 
lie in the axis), and with an incidence of moderately oblique light 
is imperceptible too, owing to the low intensity of the reflexions, 
the ordinary use of the microscope is not affected thereby in 
the least. In view, however, of the exceptional case spoken of 
above, it is expedient that the arrangement should be so disposed as 
to allow of the illuminator being replaced by the simple concave 
mirror without trouble. To ensure this, the entire diaphragm 
apparatus and the plane mirror belonging thereto form one con- 
nected piece, which by means of a groove may be slipped in and 
withdrawn under the stage of the microscope as quickly and readily 
as the condenser can be inserted into or taken out of the stage- 
plate. 
When observations are to be made with polarized light, all that 
is required is that the Nicol's prism, set in a suitable disk of metal, 
should be inserted in the diaphragm-frame instead of a stop. The 
action is in every respect similar to that in the ordinary arrange- 
ment. 
Finally, with respect to the observation of positive images of 
objects on a dark ground, nothing more is required for obtaining 
this kind of illumination, as explained above, beyond substituting in 
the diaphragm-frame, instead of one of the perforated stops, a 
narrow ring furnished with a central disk of about 12 millimeters 
in diameter, supported on thin spokes. If the angle of aperture of 
the object-glass employed is less than the angular diameter of the 
middle portion of the available surface of light below the object and 
thus rendered ineffective, no direct ray enters the microscope 
through the object, and the field remains perfectly dark. The 
image is produced exclusively by such rays as having traversed the 
object are thrown down thereon by reflexion at the limit between 
covering glass and air, and by those which on their path through 
the object are deviated in the direction of the axis, be it either by 
refraction or inflection. Either the one or the other has the pre- 
ponderance as the nature of the object may be; but when an 
immersion lens is used, of course it is only the light that is deviated 
in the object that comes into play, inasmuch as the reflexion at the 
