20 
THEORY OF THE MICROSCOPE. 
shown bj the dotted lines, they will be found to intersect the 
axis at different distances, X and Y, from the surface of the 
glass ; the distance X Y, is the aberration produced by the 
medium, which, as before stated, interferes with the previously 
balanced aberrations of the several lenses composing the object- 
glass. The spherical aberration thus produced by the thin 
glass, or other medium covering the object, being in a direc- 
tion the opposite of that produced by a single convex lens, 
is called negative aberration. Chromatic aberration is pro- 
duced by the thin glass in the same direction. The effect ob- 
served by the eye, is, that lines are not so sharply defined, and 
the outline of any object appears bordered with larger and 
thicker fringes, with colors of the secondary spectrum upon the 
borders of the object. 
33. Objective corrected for Olass Cover. If an object- 
glass constructed as shown at A M P, Fig. 5, have its anterior 
combination A, somewhat under-corrected, so as to leave a 
degree of positive aberration, and the middle and posterior 
combinations M P, have together an excess of negative aber- 
ration, so as to balance the under-correction of the anterior com- 
bination, then the positive aberration of this latter combination 
will act more powerfully upon the other two when brought as 
near to them as possible, and less powerfully when the distance 
between them is increased. When the three combinations are 
in close contact, their common focus, and consequently the 
object, is at the greatest distance from the front lens ; the rays 
from the object are therefore diverging from a j^oint at a greater 
distance than when the lenses are separated. A lens bends the 
rays more that diverge from a distant object, and of course its 
aberration is then greater, hence the anterior combination A, will 
have a greater positive aberration, and act more strongly upon 
the other combinations M and P, and this effect will vary with 
the distance between the anterior and other combinations. 
When, therefore, the correction of the entire objective is ef- 
fected for an uncovered object, with a certain distance be- 
tween the anterior and middle combinations, if they are then 
brought into contact, the distance between the anterior com- 
J. & W. GRUNOW & GO’S ILLUSTRATED 
