LIBERKUHN SPECULUM. 
63 
upwards by the plane mirror M, so as to enter the prism. It 
then suffers total reflection in 
the direction w and is again 
reflected by the opposite force 
of the prism in the direction v 
O ; as it emerges from the con- 
vex surface of the lens c? it 
is condensed and brought to a 
focus upon the object O, which 
is in the focus of the object- 
glass immediately above it. 
This prism is so mounted be- 
low the stage of the micro- 
scope that it can be revolved 
to give light from any direc- 
tion, or illuminate successive- 
ly every side of an object. 
This method of illumination 
brings out many delicate 
markings, and reveals pecu- 
liarities of structure not otherwise appreciable. See Section 16. 
When the microscope is furnished with an achromatic con- 
denser, bTachet’s prism can be inserted in the same brass work. 
84. liieberkubn Speculum. This instrument, shown in 
Fig. 32, consists of a small concave metallic reflector L L, 
attached to a short tube which is fitted to the lower part of the 
object-glass A. The polished surface presents that degree of 
concavity which is adapted to bring to a focus, upon the object, 
the beam of parallel light reflected from the plane mirror 
below the stage. These little reflectors are called Lieberkuhns, 
from the name of their inventor. 
The rays of light reflected from the mirror B, Fig. 32, pass 
through that part of the slide S S, not covered by the object, 
and being again reflected by the Lieberkuhn L L, are brought 
to a focus upon the object, which is then seen by reflected light. 
If the object is transparent, the small stop or darh well D, 
gives a black ground behind the object, which is then seen 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
