60 TRANSACTIONS OF THE CANADIAN INSTITUTE, [VoL. VIII. 
this hypothesis and that is the one discovered by Reed and Carroll in 
regard to the virus of yellow fever. 
They found that the mosquito which had become infected with the 
virus by feeding upon the yellow fever patient was not capable of trans- 
mitting the disease until at least twelve days had elapsed, and that then 
the incubation period in the infected person was five days. They also 
showed that 1.5 c.c. of the blood serum of a yellow fever patient with- 
drawn during the first two days of the disease and injected into a second 
person produced the fever in about forty-eight hours. Now it may be 
argued that the long sojourn in the body of the mosquito was necessary 
for a sufficient multiplication of the virus to produce an effective infec- 
tion or that that time was necessary for the passage through the stomach 
to the poison glands, but it was found that the bite of the infected mos- 
quito was harmless after eight or ten days, but harmful two or three days 
later, and we can hardly think that the difficulties of transit alone were 
- sufficient to account for twelve days in travelling from the stomach to the 
poison glands. The phenomenon resembles much more that which appears 
in the transmission of malaria by the mosquito. Here the length of time 
which elapses is due to the parasite undergoing a necessary cycle of its exis- 
tence in the body of the insect which results in the formation of minute 
sickle-shaped spores which then travel to the poison gland and are injected 
into the next person bitten. 
If the twelve days which the yellow fever virus passes in the body 
of the stegomyia is required for the completion of say a sexual phase of 
existence then we would have to place it in the animal kingdom as an 
ultramicroscopic form related to the malarial parasites. 
It will be seen, however, that a great deal more light must be thrown 
upon the subject before we can definitely place these invisible parasites 
in the scale of organized being, and the difficulties in the way are very 
great, first because of their minute size, second because as yet we do not know 
how to cultivate them, and thirdly because they are all parasites and 
consequently can only be studied in the living organism. 
In regard to their minute size the question may be asked, must they 
be definitely placed for all time outside the range of microscopic vision or 
may our optical equipment yet develop to such an extent that we may 
demonstrate their organism and even smaller particles? 
The theoretical limit of the power of the microscope to demonstrate 
structure is about 0.25 micron. 7.e., one half a wave length of the middle 
part of the spectrum. Below this magnitude minute particles will no 
longer show structure but will appear as diffraction discs. The smallest 
visible particle with the highest power of the microscope, structure being 
