as seldom, if ever,
been equalled. Nor were his efforts fruitless, as a single example will
suffice to show. By his 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 perplexities are introduced by the fact that the use
of a wide pencil of light is a desideratum, in order to gain sufficient
illumination when large magnification is to be secured.

In the attempt to overcome those difficulties, the foremost physical
philosophers of the time came to the aid of the best opticians. Very
early in the century, Dr. (afterwards Sir David) Brewster, the renowned
Scotch physicist, suggested that certain advantages might accrue from
the use of such gems as have high refractive and low dispersive indices,
in place of lenses made of glass. Accordingly lenses were made of
diamond, of sapphire, and so on, and with some measure of success. B