n] 



THE LEAST OF ORGANISMS 



39 



the thinnest gold-leaf ; and as small if not smaller still are a few 

 bacteria and their spores. But here we have reached, or all but 

 reached the utmost limits of ordinary microscopic vision; and 

 there remain still smaller organisms, the so-called "filter-passers," 

 which the ultra-microscope reveals, but which are mainly brought 

 within our ken only by the maladies, such as hydrophobia, foot- 

 and-mouth disease, or the "mosaic" disease of the tobacco-plant, 

 to which these invisible micro-organisms give rise*. Accordingly, 



B 



B 



Fig. 2. Relative magnitudes of: A, human blood-corpuscle (7-5^1 in diameter); 

 B, Bacillus anthracis {4: — 15 fx x I /j.) ; C. various Micrococci (diam. 0-5 — l/n, 

 rarely 2^i); D, Micromonas progrediens, Schroter (diam. 0-15/x). 



since it is only by the diseases which they occasion that these 

 tiny bodies are made known to us, we might be tempted to 

 suppose that innumerable other invisible organisms, smaller and 

 yet smaller, exist unseen and unrecognised by man. 



To illustrate some of these small magnitudes I have adapted 

 the preceding diagram from one given by Zsigmondyf . Upon the 



* Recent important researches suggest that such ultra-minute '"filter-passers" 

 are the true cause of certain acute maladies commonly ascribed to the presence 

 of much larger organisms; cf. Hort, Lakin and Benians, The true infective 

 Agent in Cerebrospinal Fever, etc., J. Roy. Army Med. Corps, Feb. 1916. 



■(• Zur Erkenntniss cler Kolloide, 1905, p. 122; where there wiU be found an 

 interesting discussion of various molecular and other minute magnitudes. 



