174 
LEROY F. THOMPSON. 
geneous oil immersion objective, the compensating ocular and 
Abbe condenser are in constant use in the laboratory and enable 
us to secure readily magnification to 1,500 diameters or more. 
History has shown that in the last several decades there has 
been little increase in magnification due to two principal reasons. 
The greater the magnification the more convex and consequently 
the smaller must be the lenses used in the objectives and more 
difficult becomes their grinding and adjustment. Furthermore, 
the physicist tells us that a clear view with determination of the 
size and shape of microscopic objects cannot be obtained when 
the objects examined are smaller than one-half the wave length 
of the rays of light in which they are examined. 
There is thus a seemingly insurmountable barrier set to an 
indefinite increase in magnification. A recent advance has been 
made through the development of the ultra-microscope, which, 
I hope, will do away with that term given when you are asked 
to give the specific cause of a given disease and of which we are 
all so familiar (to wit) : It is caused either by the ultra-micro¬ 
scopic organism or a filterable virus, as in the case of hog cholera, 
and, of course, covers a multitude of sins due to our lack of 
knowledge along that line. 
The ultra-microscope has made visible objects much smaller 
than those which had been previously observed. A bright gleam 
of light from an arc or similar source is passed across the dark¬ 
ened field of the microscope and the light is reflected to the eye 
from any particles that may be in suspension. These objects 
are seen in the same manner that minute particles of dust are 
made visible in a bright ray of light that enters a darkened room. 
The use of the ultra-microscope has not as yet added many facts 
of value to our knowledge of the bacteria. 
Leeuwenhoek, the first observer of bacteria, contributed very 
little to the knowledge of their essential nature. F. Muller in 
1786 worked out a simple classification, but did not differentiate 
between bacteria and protozoa. To him we owe several of the 
group names applied to bacteria, such as Bacillus, Vibrio and 
Spirillum. 
