64 : 
ACCESSORY APPARATUS. 
brilliantly illuminated upon a 
dark field. The dark well is 
made in the form of a cup, in 
order that the bottom may 
not be sufficiently illuminated 
to form a light ground to the 
object, which might happen 
if the disk were employed. 
Three sizes of these dark 
wells are usually supplied, the 
largest being always used 
with the lower powers. 
The Lieberkuhn gives a 
full and uniform illumination 
on every side of the object, 
while the bull’s-eye gives 
only oblique light from one 
direction. The Lieberkuhn 
is also available to illuminate 
opaque objects viewed with object-glasses of such short focus 
as to preclude the use of the bull’s-eye. 
85. The Erector is inserted in the lower end of the draw- 
tube of the microscope. Its use is to reverse the position of 
the image, (which is inverted in the compound microscope.) so 
that it shall appear in the true position of the object. It is 
used with low powers for dissecting and other manipulations, 
where the hands require to be guided by looking through the 
microscope. 
86. Among the recent valuable additions to the microscope, 
should be mentioned the Ortlioscopic Eye-piece, invented by 
Mr. Charles Kellner, optician of Wetzlar. It is adapted to micro- 
scopes, and also to telescopes of all kinds, the dialy tic included. 
It gives a large field of view, free from curvature or distortion 
of any kind, perspectively correct, with sharpness of definition 
throughout its whole extent, without the blue ring which en- 
circles the borders of the field in the ordinary negative eye-piece. 
Fig. 32. 
J. & W. GRUNOW & GO’S ILLUSTRATED 
