62 
ACCESSORY APPARATUS. 
which carries the condenser, is moved by rack and pinion by 
turning the milled head, which serves to adjust the focus. 
The achromatic condenser fits to the inner tube by a bayonet 
joint, and when it is removed, a diaphragm, or N'achefs 
Oblique Prism^ may be inserted in the same brass work. 
The achromatic condenser furnishes a pencil of light which is, 
1st, free from color ; 2nd, free from spherical aberration ; 3rd, 
condensed at a very large angle. These qualities render it 
exceedingly valuable for displaying delicate objects viewed 
with the higher powers. 
To use this condenser, first place an object on the stage, 
illuminating it by means of the concave mirror, and with a low 
power bring the object into exact focus; now remove the 
object without altering the focus of the object-glass, attach the 
achromatic condenser and illuminate it by means of the plane 
mirror; now by turning the milled head, so adjust the con- 
denser that the image of window bars or trees may be seen in 
the microscope ; then turn the mirror so as to reflect light from 
a white cloud, or other strong light, through the condenser ; 
then place the object on the stage and attach one of the higher 
powers to the microscope, and when the object is brought into 
focus it will be seen most beautifully illuminated, and delicate 
structures, scarcely distinguishable by other methods of illumi- 
nation, will be clearly defined. 
83. ]Vactiet’§ Prii^m for OMique Illumiiiatiou. This 
prism is so contrived as to illuminate the object with a pencil 
of light from one side of the axis, the form of the prism being 
previously constructed so as to give to the light any angle of 
inclination required. Let M O, Fig. 31, be the axis of the 
microscope, h c d q are the four sides of ITachet’s 
prism, 'M? O is the angle at which the pencil of light is re- 
quired to fall upon the object O. To the side d q^ is cemented 
a lens, having such a focus as to condense the light, transmitted 
by the prism, upon the object O, on the stage of the micro- 
scope. This prism is set in a piece of tube C C, a diaphragm 
placed below cuts off all the light whicli would not pass 
through the prism and be brought to a focus at O. 
When this prism is used a beam of light L, is reflected 
J. & W. GRUNOW & GO’S ILLUSTRATED 
