330 LEEUWENHOEK AND HIS ‘‘ LITTLE ANIMALS” 
screw passes through the stage near one end [figs. 1, 2,4], and 
presses vertically against the plates, causing the stage to tilt 
up at that end; the fitting of the long screw-carrier (angle- 
piece [fig. 3]) is such that the stage at the end is sprung 
down somewhat forcibly on the brass plates, and it is against 
this pressure that the focussing screw acts.” 
As we have already heard, the object to be viewed was 
either stuck directly on to the pin of the object-carrier, or 
else it was first mounted in some way (¢.g.,on a small plate of 
glass or mica, or between two thin glass plates like a modern 
slide and coverslip, or in a capillary glass tube) and the whole 
was then fixed and focussed before the lens. A “ microscope” 
of this type, with a lens of very short focus and high magni- 
fication, must have been extremely awkward to manipulate. 
It would be necessary to place the eye almost in contact with 
the lens, and it is not clear how Leeuwenhoek was able to 
obtain the requisite illumination. The known magnifying 
power of his best glasses was, of course, sufficient to enlarge 
objects as small as blood-corpuscles (and even bacteria) to 
visually perceptible dimensions—a fact which modern workers 
with the compound microscope seem apt to overlook. With 
the front lens of my 2 mm. apochromatic objective—having a 
magnification of about 120 diameters—I can distinctly see 
(using the light of a clear sky only) bacteria as small as 
Bacillus coli in a stained film. But to see such organisms 
alive in water, and with sufficient clarity to describe their 
movements, is another matter. (My own eyesight, I should 
note, is exceptionally good—probably but little inferior to 
Leeuwenhoek’s.) Yet Leeuwenhoek not only knew how to 
make lenses of adequate magnifying power and aperture and 
resolution, and sufficiently free from spherical and chromatic 
aberration, but he also understood how to obtain the necessary 
visibility. He was able,in some way, to get the indispensable 
contrast between the object and its background which we 
now readily obtain by means of central stops, iris diaphragms, 
or staining. 
It appears to me certain, indeed, that Leeuwenhoek cannot | 
have made his extraordinarily accurate observations on bac- 
teria and protozoa by means of the apparatus just described 
when used in the ordinary way. He had unbounded patience 
and magnificent eyesight—as his works abundantly testify— 
