30 BACTERIOLOGICAL APPARATUS 
character, i.c., nearer the yellow and blue. Apochromatic objectives 
show no color borders in this test. 
The spherical correction of an objective is perfected for a certain 
thickness of cover glass and a certain tube length, and is influenced 
greatly by any variation in either. This is especially true with the 
high-power dry objectives. The homogeneous immersion objectives 
are not sensitive to the variation in the cover thickness, because the 
immersion oil between the cover glass and the lens is of the same refrac- 
tive index as the glass. They must be used, however, with the proper 
tube length. In testing an objective for its spherical correction it is 
therefore very Important to supply the proper thickness of cover and 
tube length. It is manifestly unfair to judge an objective on this 
point without complying with these conditions. The test for spherical 
correction can be made on the same object as used for the chromatic 
test. If the edges of the lines in the center of the field appear equally 
sharp and clear when illuminated by either a narrow central cone of 
light or a narrow oblique cone without having to change the fine adjust- 
ment the objective is spherically corrected. The color remnants men- 
tioned above will be clear and transparent, while if the lens is poorly 
corrected spherically, these borders will appear muddy and turbid. 
Defects in spherical corrections can often be corrected by using cover 
glasses suitable to them, also by changing the tube length. The fact 
that the periphery of the field is not in focus at the same time as the 
center does not bespeak a lack of spherical correction, but a lack of 
flatness of field with which it is often confused. 
Flainess of field depends not only upon the objective itself, but upon 
the ocular and the cone of light used, whereas the spherical aberration 
is inherent in the objective itself. No field is absolutely flat. It is a 
desirable quality in a lens, but spherical and chromatic corrections 
should never be sacrificed for it. Some lenses appear to be “‘ flatter ”’ 
than they really are, because their corrections are so poor that little 
contrast 1s noticed between objects in the center of the field and 
at the edge. Narrow cones of light give a flatter field than wide 
ones. Thin objects are more critical tests for flatness of field than 
thick ones. 
Working distance is the free distance between the cover glass and 
the objective when the latter is focused. It decreases generally with 
increasing power and numerical aperture of the objective. Of two lenses 
with the same focal distance the one with the higher N.A. will have 
the shorter working distance. The working distance also depends 
on the mounting of the front lens. If the lens has a prominent mount- 
