INVISIBLE VIRUSES 169 



organisms are sometimes termed " filter passers." These 

 experiments do not necessarily prove that the organism 

 in all stages is invisible. Hort claims that many of the 

 ordinary bacteria form minute buds which may pass 

 through the larger-pored filters (loc. cit. p. 13), and 

 Noguchi has demonstrated a minute spirochaetal form in 

 yellow fever. Besides invisibility and filtrability, these 

 ultra-microscopic viruses present certain common features. 

 They are all destroyed at a comparatively low tempera- 

 ture and by most weak antiseptics. On the other hand, 

 they maintain their vitality for a considerable time in 

 pure glycerin. They are generally conveyed only by 

 contact, by inoculation, or by insect intermediaries, and 

 not by the air, water or soil, and only exceptionally by 

 fomites. The pathological lesions are similar and charac- 

 terised by cell-inclusions and alterations of the cellular 

 nuclei. 



Microscopical Measurements 



The measurement of micro-organisms is carried out by 

 means of a stage micrometer, alone, or in combination 

 with an eye-piece micrometer. The former consists of 

 a scale of tenths and hundredths of a millimetre or hun- 

 dredths and thousandths of an inch ruled in fine lines on 

 a glass plate, by means of which the measurements can 

 be made by focussing the scale under the microscope. 

 The stage micrometer is placed in position on the stage 

 and the scale is focussed with the particular ocular, objec- 

 tive, and tube-length which are to be used. A drawing 

 of the scale is made with a camera lucida ; the micro- 

 meter is then removed and the object placed in position 

 and a second drawing is made of the object on the scale 

 already drawn. A simpler and less expensive arrange- 

 ment is to make use of a disc of glass ruled with equi- 

 distant fine lines, which can be placed on the diaphragm 



