168 A MANUAL OF BACTERIOLOGY 



specimen, and a higher power ( x 8 to x 18) for observa- 

 tional purposes. If apochromatic objectives be used the 

 compensating oculars will, of course, be employed. 



Work with dark-ground illumination should be carried 

 out in a darkened, though not dark, room, or at least in 

 the darkest corner of the room, and screened from any 

 direct light other than the illuminant used. 



Ultra -microscopic Organisms 



As already mentioned (p. 154), objects having a dia- 

 meter of less than about 0-16 ju, cannot be seen with the 

 best optical appliances and using monochromatic light of 

 short wave-length (by which resolution is increased to 

 146,000 lines to the inch). If, then, a micro-organism is 

 less in size than this it could not be seen microscopically, 

 and this fact may explain why it is that in certain 

 undoubted infective diseases no micro-organism has yet 

 been detected. Of the existence of such " ultra-micro- 

 scopic " organisms we have proof. The finest porcelain 

 filters, such as the Chamberland B, do not allow visible 

 particles to pass through, yet in several instances, if the 

 infective material be filtered through such a filter, the 

 filtrate is still infective. This is the case with the blood- 

 serum in yellow fever, trench fever, Cape horse sickness, 

 dog distemper, hog cholera, and swine fever, in bird and 

 cattle plagues, and with the juice of bird molluscum. 

 The organism of cattle pleuro-pneumonia is just on the 

 limit of visibility. The rabic and vaccine viruses also 

 seem capable of passing through a Berkfeld V. Some 

 thirty ultra-microscopic viruses are now known, including, 

 in addition to those mentioned above, those of anterior 

 poliomyelitis, measles, molluscum, and trachoma. 1 These 



1 See Roux, Bull, de VInst. Past., vol. i, 1903, pp. 1 and 49. Remlinger, 

 ibid. vol. iv, 1906, pp. 337 and 385; Trans. XV I Ith Internal. Cong. Med., 

 1913, Sect. IV, Pt. I, pp. 35 (Loffler) and 49 (McFadyean). 





