2 
THEOEY OF THE MICROSCOPE. 
prineipal focus. If a pencil of light diverging from a point 
behind the lens, as F', more distant from the lens than the 
Fig. 1. 
principal focus, falls upon the lens, it will be refracted to a focus 
f, also more distant than the principal focus. When F' ap- 
proaches the lens, f will recede from it, and vice versa., hence 
these two are called the conjugate foci of the lens. The rela- 
tive distances of the conjugate foci have a certain relation to 
the distance of the principal focus F, 
4. Aberration. In a perfect lens the above statements 
would be strictly correct, but in all spherical lenses it is found, 
on careful examination, that the rays falling upon the lens at 
different distances from the centre, do not all meet in a single 
point, but are subject to two different causes of error, called 
spherical and chromatic aberration, which will now be ex- 
plained. 
Fig. 2. 
5. Spherical Aberration. Let F, Fig. 2, represent tlie 
focus of two parallel rays L L, near the centre of the lens ; 
then if 1 1 represent the rays falling upon the borders of the 
lens, they will be so refracted as to meet at a point f, con- 
J. & W. GRUNOW & GO’S ILLUSTRATED 
