290 Transactions of the Society. 



X. — On the Measurement of the Diameter of the Flagella of 

 the Cholera Bacillus prepared by Lojflers Method. 



By A. A. C. Eliot Merlin. 



(Read March 16, 1910.) 



Slides of bacteria intended to render the flagella demonstrable 

 even under the highest powers of the modern Microscope are 

 almost invariably prepared by Loffler's, or some similar method, 

 which greatly distends the organism and its appendages, thus 

 rendering them comparatively coarse objects. It has been said 

 that the reason why flagella are (supposedly) invisible in the 

 ordinary balsam- stained preparations is because these appendages 

 then fall alongside the microbes. As a matter of fact, it has long 

 been known that flagella are readily observable in such slides 

 under proper optical conditions. 



Little or nothing seems to have been attempted as regards the 

 measurement of the diameter of these delicate appendages since 

 the late Dr. Dallinger read his famous paper " On the Measure- 

 ment of the Diameter of the Flagella of Bacterium termo : a 

 Contribution to the Question of the ' Ultimate Limit of Vision ' 

 with our present Lenses." * Dr. Dallinger made four sets of fifty 

 separate drawings and measurements with each of four lenses 

 GV' i J> 55> an( ^ 35 ^ n -)> an( ^ taking the mean of these two hundred 

 measurements found ■ 00000488526, or nearly 2 oiV oo * n -> ^° ^ e 

 the value of the diameter of the flagellum of B. termo. We now 

 know that this value requires antipoint correction, which makes 

 the true diameter of the flagellum 0-00000993 ( TT5 ^ 00 in -)> 

 assuming that the W.A. was • 8.f In truth, a flagellum possess- 

 ing such a thickness should prove readily visible with a X.A. of 

 0*5, and thus be well within the grasp of an ordinary good £ in. 

 of the period (my Powell \ in., made in 1850, has N.A. ■ 72, and 

 will stand a large W.A.). Nevertheless, we can imagine the incredu- 

 lity that would have found open expression had anyone calmly 

 stated at the time that the flagella of B. termo could be readily 

 perceived with such an objective ! 



At the present day lenses of N.A. 1 ■ 3 and 1*4, in the hands 

 of trained professional bacteriologists, are apparently insufficient 

 to demonstrate the existence of the flagella of B. tuberculosis and 

 M. melitensis, although both are visible under proper optical con- 

 ditions with a dry objective in ordinary balsam well-stained slides. 



* See this Journal, 1878, p. 169. f Op. cit., 1903, p. 581. 



