14 
THEOKY OF THE MICROSCOPE. 
the light, except that required for the formation of a perfect 
image, and to limit the field of view to such an aperture as 
will be well defined when viewed with the eye-lens E E. 
The image formed at B B, is further magnified by the eye- 
lens, as if it were an original object. The triple achromatic 
object-glass, constructed on the principles discovered by Mr. 
Lister, though capable of transmitting large angular pencils, 
and corrected as to its own errors of spherical and chromatic 
aberration, would, nevertheless, be incomplete without some 
special adaptation to prevent renewed aberrations distorting the 
image as transmitted and viewed by the eye-piece. 
The property of the negative eye-piece, pointed out by Bos- 
covich, most admirably meets this condition ; for although it 
would not be free from aberrations when used alone, yet, when 
used as an eye-piece, in connection with an objective, its effect 
upon converging pencils is such that the aberrations produced 
by the field-lens are very nearly balanced by opposite aberra- 
tions in the eye-lens, resulting from the fact that it is situated 
on the opposite side of the image formed between the two 
lenses, and because the light from the object falls only upon 
those parts of the field-lens F F, which are best adapted to 
transmit it free from error. 
27. Enlarged Section of tlie Compound Microscope. A 
more complete view of the action of the several parts of the 
compound achromatic microscope, is given in Fig. 5, the lenses 
being represented on an enlarged scale. A A, M M, P P, repre- 
sent the three compound lenses of which the achromatic ob- 
jective is composed. F F is the field-lens, and E E the eye-lens 
of the negative eye-piece. 
Three raj^s drawn from the centre of the object O, and three 
from each extremity, show the course of both direct and 
oblique pencils. It is impossible in the space allowed to the 
figure, to show the separate action of each concave and convex 
lens, but only the action of the objective considered as a whole. 
First : the axial rays, both of direct and oblique pencils, 
cross at some point which constitutes the optical center of the 
compound objective, and emerge from the posterior lens in the 
same direction they pursued on leaving the object ; the lateral 
J. & W. GRUNOW & GO’S CATALOGUE. 
