e, or if the
tube were bent in the shape of a _U_ and the end left open,
organisms from the air could not pass into the tube against gravity
when air movement within the tube was prevented by bending. The
possibility of spontaneous generation cannot be denied, but that it
takes place is against all human experience.

It was not possible to attain any considerable knowledge of the
bacteria discovered by Loewenhoeck until more perfect instruments for
studying them were devised. Lenses for studying objects were used in
remote antiquity, but the compound microscope in which the image made
by the lens is further magnified was not discovered until 1605, and
when first made was so imperfect that the best simple lenses gave
clearer definition. With the betterment of the microscope, increasing
the magnifying power and the sharpness of the image of the object
seen, it became possible to classify the minute organisms according to
size and form and to study the separate species. The microscope has
now reached such a degree of perfection that objects smaller than one
one hundred thousandth of an inch in diameter can be clearly seen and
photographed.

Great impetus was given to the biological investigation of disease by
the discoveries which led to the formulation of the cell theory in
1840 and the brilliant work of Pasteur on fermentation,[1] but it was
not until 1878 that it was definitely proved that a disease of cattle
called anthrax was due to a species of bacteria. What should be
regarded as such proof had been formulated by Henle in 1840. To prove
that a certain sort of organism when found associated with a disease
is the cause of the disease, three things are necessary:

1. The organism must always be found in the diseased animal and
associated with the changes produced by the disease.

2. The organism so found must be grown outside of the body in what is
termed pure cultures, that is, not associated with any other
organisms, and for so long a time with constant transfers or new
seedings that there can be no admixture of other products of the
disease in the material in which it is grown.

3. The disease must be produced by inoculating a susceptible animal
with a small portion of such a culture, and the organism shown in
relation to the lesions so produced.

It is worth while to devote some attention to the disease anthrax.
This occupies a unique position, in that it was the first of the
infectious diseases to be scien