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History of Science, a — Volume 4 by Henry Smith Williams;Edward Huntington Williams
page 105 of 296 (35%)
examinations he was able to prove that diseases of the chest,
which had formerly been classed under the indefinite name
"peripneumonia," might involve three different structures, the
pleural sac covering the lungs, the lung itself, and the
bronchial tubes, the diseases affecting these organs being known
respectively as pleuritis, pneumonia, and bronchitis, each one
differing from the others as to prognosis and treatment. The
advantage of such an exact classification needs no demonstration.


LISTER AND THE PERFECTED MICROSCOPE

At the same time when these broad macroscopical distinctions were
being drawn there were other workers who were striving to go even
deeper into the intricacies of the animal mechanism with the aid
of the microscope. This undertaking, however, was beset with
very great optical difficulties, and for a long time little
advance was made upon the work of preceding generations. Two
great optical barriers, known technically as spherical and
chromatic aberration--the one due to a failure of the rays of
light to fall all in one plane when focalized through a lens, the
other due to the dispersive action of the lens in breaking the
white light into prismatic colors--confronted the makers of
microscopic lenses, and seemed all but insuperable. The making of
achromatic lenses for telescopes had been accomplished, it is
true, by Dolland in the previous century, by the union of lenses
of crown glass with those of flint glass, these two materials
having different indices of refraction and dispersion. But, aside
from the mechanical difficulties which arise when the lens is of
the minute dimensions required for use with the microscope, other
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