Flagella of Cholera Bacillus. By A. A. C. E. Merlin. 291 



Under these circumstances it has been thought that a measure- 

 ment of the diameters of the cholera bacillus flagella, prepared by 

 Loftier' s method, may be of service if only to render manifest the 

 real dimensions of appendages commonly considered delicate objects 

 entailing the employment of the highest powers for their success- 

 ful demonstration. Kecent advances in practical microscopy render 

 it easy to ascertain the true diameter of flagella with great precision 

 and little labour ; a suitable objective, with a Koyston-Pigott * iris 

 diaphragm above it, and an Abbe apertometer being needed for 

 the purpose. In the present instance extinction measurements f 

 were effected with an apochromatic £ in. of 20-2 I.M.P. and N.A. 

 - 70, and an apochromatic nominal § in. of 18*6 I.M.P. and N.A. 

 0"35. With both these lenses it was found that the very finest 

 flagella in the Loffler slide were extinguished with a clear N.A. of 

 0*25 and the coarser at N.A. 0*24, a full illuminating cone and 

 Clifford F line screen being employed. It is thus apparent by the 

 table on p. 550 of the Journal (1909) that the finest flagella pos- 

 sess a diameter of ■ 00001545 (64725) in -> tne coarser equalling 

 0' 00001606 (g2266) m - Some lew of the coarsest appendages 

 even slightly exceed the latter diameter. All the flagella are 

 beautifully shown under a semi -apochromatic 1 in. of 12 # 1 I.M.P. 

 and N.A. • 28, used critically with a screen. 



An excessively slight closing of the Pigott diaphragm produces 

 invisibility of the flagella when the critical point is reached. It 

 is necessary that the clear N.A. should be very exactly ascertained 

 by means of the Abbe apertometer preferably used on a revolving 

 divided stage as described in " Carpenter " (eighth edition, p. 395), 

 and fully explained in this Journal (1896, p. 592). For the 

 definite extinction of most structures it will be found necessary to 

 reduce the N.A. in the manner described, i.e. by employi: , a full 

 illuminating cone and cutting down the objective's aperture by 

 means of a diaphragm placed immediately above it. The proper 

 extinction point cannot be exactly ascertained with an objective of 

 large N.A. by merely closing the condenser diaphragm and thus 

 reducing its W.A., it having been found in practice that the 

 presence of flagella is indicated, when this method is employed, by 

 vague diffraction effects long after the true extinction point is 

 passed. It is, therefore, necessary or advisable to always substitute 

 clear N.A. for W.A. in effecting extinction measurements by the 

 table given in this Journal (1909, p. 550). I am able to state 

 that Mr. Nelson fully concurs in this view. 



Addendum. 



As a check on the accuracy of the above described extinction 

 measurements, it was thought desirable to ascertain, if possible, at 



* Monthly Micr. Journ., xiii. (1875) p. 56. 

 f See this Journal, 1909, p. 549. 



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