= 
I.—PHYSIOLOGY. 175 
But the strict application of such a definition, based on negative character- 
istics, must obviously narrow its scope with the advance of technique. We 
_ may look a little more closely at the meaning of these different characters, 
concerning one or another of which those who follow me will, doubtless, 
have more to say in detail. 
Microscopic visibility is obviously a loose term. Rayleigh’s familiar 
formula, in which the lower limit of resolution is equal to one-half the 
wave-length of the light employed, divided by the numerical aperture of 
_ the objective, only gives us the smallest dimensions of an object, of which, 
with the method of transmitted illumination habitually used in former 
_ years, a critical image can be formed. There can be no doubt that the 
_ separate particles, of practically all the agents to which the term virus 
would be applied, fall below this limit of size. To put it in plain figures, 
their diameter is less than 0°2 micron. On the other hand, progress 
has recently been, and continues to be,rapid in the direction of bring- 
ing into the visible range, minute bodies associated with a growing 
_ number of viruses. This has been effected, on the one hand, by improve- 
ment in staining technique, which probably owes its success largely to 
increase of the natural size of the particles by a deposit of dye on their 
surface; and, on the other hand, by forming visible diffraction images 
_ of the unstained particles with wide-aperture dark-ground condensers, and 
_ by photographing the images formed of them with shorter, invisible rays. 
Mr. Barnard will present evidence for success in obtaining such sharp 
_ photographic images of the bodies associated with one virus, measurements 
_ of which give their natural size by simple calculation. The reaction of a 
cautious criticism to such a demonstration seems to have taken two 
different directions. There has been a tendency, on the one hand, to 
exclude an agent from the group of viruses as soon as the microscope 
could demonstrate it with some certainty. Many have for years thus 
excluded the agent transmitting the pleuropneumonia of cattle, though 
the status of this organism has been compromised even more by the 
success of its cultivation on artificial media. Visibility seems to have 
tendered doubtful the position of the Rickettsia group of infections, 
and, if the test is logically applied, the process of exclusion can hardly 
stop before the agents transmitting psittacosis, fowl-pox, infectious 
ectromelia, and even vaccinia and variola, have been removed from the 
‘group of viruses into that of visible organisms. In discussing the 
biological nature of viruses as a whole, however, we can hardly begin by 
accepting an artificial and shifting limitation of that kind. The real task 
before us, rather, is to discuss to what extent the evidence of these recent 
developments, which appear to show that some of the agents, known 
hitherto as viruses, consist of very minute organisms, can safely be applied 
to other viruses, which are still beyond the range of resolution. Do these 
also consist of organisms still more minute, or are any of them unorganised ? 
Another line of criticism, sound in itself, while not excluding from the 
virus group these agents for which microscopic visibility has been claimed, 
demands more evidence that the minute bodies seen or photographed are 
Teally the infective agent, and not merely products of a perverted 
metabolism which its presence engenders. It is obvious that complete 
