ULTRA-MICROSCOPIC ORGANISMS 141 



addition to resolving power, not much is gained, in 

 ordinary work at any rate, by adopting the immersion 

 system for the lower power objectives, such as the g-in. 



By the physical theory of microscopical visibility, it can be 

 shown that objects having a diameter of less than about 0-16 p 

 cannot be seen with the best optical appliances. 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 isolated. Of the 

 existence of such " ultra-microscopic " 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, Cape horse sickness, dog distemper, hog cholera, and swine 

 fever, in bird and cattle plagues, and with the juice of bird mollus- 

 cum. 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. These experiments do not neces- 

 sarily prove that the organism in all stages is invisible. l Siedentopf 

 and Zsigmondy have devised a method whereby ultra-microscopical 

 particles may be rendered visible, but inasmuch as they appear 

 merely as luminous points, it is questionable whether the method 

 will be of great service in bacteriology. Some thirty ultra - 

 microscopic viruses are now known, including, in addition to those 

 mentioned above, those of anterior poliomyelitis, measles, mol- 

 luscum, and trachoma. 



There is no real necessity in bacteriological work for the 

 immersion objective to be provided with a " correction 

 collar." The " correction collar " is an additional screw 

 in the objective by means of which the distance between 

 some of its constituent lenses can be altered to " correct " 

 for varying thicknesses of cover- glass, etc., and though 

 necessary with the higher power dry lenses, it is theo- 

 retically unnecessary with the immersion system. Never- 

 theless, as slight variations do occur in the various media, 



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

 linger, ibid. vol. iv, 1906, pp. 337 and 385; Trans. XVIIth Internal. 

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



