period. Hitherto, ever
since the discovery of magnifying-glasses, there had been here and there
a man, such as Leuwenhoek or Malpighi, gifted with exceptional vision,
and perhaps unusually happy in his conjectures, who made important
contributions to the knowledge of the minute structure of organic
tissues; but now of a sudden it became possible for the veriest tyro to
confirm or refute the laborious observations of these pioneers, while
the skilled observer could step easily beyond the barriers of vision
that hitherto were quite impassable. And so, naturally enough, the
physiologists of the fourth decade of the nineteenth century rushed
as eagerly into the new realm of the microscope as, for example, their
successors of to-day are exploring the realm of the X-ray.

Lister himself, who had become an eager interrogator of the instrument
he had perfected, made many important discoveries, the most notable
being his final settlement of the long-mooted question as to the true
form of the red corpuscles of the human blood. In reality, as everybody
knows nowadays, these are biconcave disks, but owing to their peculiar
figure it is easily possible to misinterpret the appearances they
present when seen through a poor lens, and though Dr. Thomas Young and
various other observers had come very near the truth regarding them,
unanimity of opinion was possible only after the verdict of the
perfected microscope was given.

These blood corpuscles are so infinitesimal in size that something like
five millions of them are found in each cubic millimetre of the blood,
yet they are isolated particles, each having, so to speak, its own
personality. This, of course, had been known to microscopists since the
days of the earliest lenses. It had been noticed, too, by here and
there an observer, that certain of the solid tissues seemed to present
something of a granular texture, as if they, too, in their ultimate
constitution, were made up of particles. And now, as better and better
lenses were constructed, this idea gained ground constantly, though
for a time no one saw its full significance. In the case of vegetable
tissues, indeed, the fact that little particles encased a membranous
covering, and called cells, are the ultimate visible units of structure
had long been known. But it was supposed that animal tissues differed
radically from this construction. The elementary particles of vegetables
"were regarded to a certain extent as individuals