10 
THEORY OF THE MICROSCOPE. 
throughout. Third : that the light admitted should, as much as 
possible, be only such as goes to form the image, and it should 
not be intercepted or diffused over the field by too many 
reflections. 
19. Mr. liister’s two preliminary propositions are : 
First : that if a piano concave lens of flint glass is employed to 
correct the aberrations of a double convex lens of crown glass, 
the correct centering of the lenses is more easily effected, and 
accurate workmanship for a short focus is much simplified ; 
though other forms may be employed for special purposes. Sec- 
ond : that the concave and convex lenses should be cemented to- 
gether with some substance permanently homogeneous, and the 
two curves should be identical in form, and pressed close 
together, so as to leave between them but a very thin layer of 
cement. By cementing the lenses in this manner, the loss of 
light by reflection is diminished by nearly one half. Third, 
that the compound piano convex lens thus formed, with curves 
which render it nearly free from aberration, should always be 
used with its flat surface towards the object to be examined. 
20. Tlie principles discovered by Mr. leister are briefly 
stated as follows : 
In lenses formed as above mentioned, with a piano concave 
lens of flint-glass cemented to a double convex lens of crown, 
rendered achromatic by proper adjustment of the curves of the 
two lenses, there is some point not far from the principal focus, 
on the plane side of the compound lens, and situated in its 
axis^ from which light falling upon the lens is transmitted free 
also from spherical aberration, and emerging either nearly 
parallel, or tending to a conjugate focus within the tube of a 
microscope. If the radiant point is brought nearer to the lens, 
the spherical aberration will be over-corrected ; but if the radi- 
ant continues to approach the lens, another point will be found 
for which the spherical aberration is again exactly balanced. 
For every radiant point still nearer to the lens, or more distant 
than the flrst point, the spherical aberration will be under- 
corrected. The two radiant points for which the lens is per- 
fectly corrected, both for chromatic and spherical aberration, 
J. k W. GRUNOW & GO’S ILLUSTRATED 
