ACHROMATIC CONDENSER. 
61 
82. Aclironiatic Condenser. For low powers the concave 
mirror, placed below the stage, furnishes all the light which is 
required, but for developing the best effect of the higher pow- 
ers, as the J, J and j 2 i^^^h objectives, it is sometimes necessary 
to have the object illuminated with achromatic light highly 
concentrated. If the object-glass has an angular aperture of 
from ninety to one hundred and sixty degrees, and the pencil 
of light by which the object is illuminated is condensed at an 
angle of only fifty degrees, (and especially if it is also affected 
with spherical and chromatic aberration,) it is evident that 
unless the object itself disperses the light, there will be no 
light to be taken up by the marginal part of the object-glass. 
It is found that generally objects do thus disperse the light to a 
limited extent, but that to secure the fullest advantage from 
object-glasses of large angular aperture, it is necessary to illumi- 
nate the object with a pencil of achromatic light, condensed at 
an angle bearing a considerable proportion to the aperture of 
the object-glass. 
This object is secured by passing the illuminating pencil 
through an achromatic combination of lenses, of large aper- 
ture, placed beneath the stage. 
Fig. 30. 
The achromatic condenser is so mounted that its focus can be 
easily adjusted to the exact position of the object. The appa- 
ratus in which it is mounted, is shown in Fig. 30, and is 
attached to the under side of the stage by a bayonet catch 
shown in the upper part of the figure. An inner cylinder, 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
