A. A C. ELIOT MERLIN ON THE MINIMUM VISIBLE. 391 



been observed by many microscopists, including myself, and has 

 been beautifully photographed by Mr. Nelson.* 



Now the flagellum of B. termo was most carefully measured 

 by the late Dr. Dallinger, and his results were embodied 

 in a paper entitled " On the Measurement of the Diameter 

 of the Flagella of Bacterium termo : a Contribution to the 

 Question of the ' Ultimate Limit of Vision ' with our Present 

 Lenses." f Two hundred measurements were made by means of 

 a fine pencil mark made over half or two-thirds, not over the 

 whole, of the camera-lucida image of the flagellum. The labour 

 entailed may be judged from Dr. Dallinger's statement: "Now 

 I made fifty separate drawings and measurements with each 

 of the four lenses, the same conditions being observed in each 

 case. The results expressed in decimal fractions are as follows, 

 viz. : 



" 1. The mean value of fifty measurements made with the 

 1/1 2th in. objective gives for the diameter of the flagellum 

 000000489208. 



" 2. The mean value of fifty measurements made with the 

 l/16th in. objective gives 0-00000488673. 



" 3. The mean value of fifty measurements made with the 

 l/25th in. objective gives 0-00000488024. 



" 4. The mean value of fifty measurements made with the 

 l/35th in. objective gives 0-00000488200. 



" We thus obtain a mean from the whole four sets of measure- 

 ments, which gives for the value of the diameter of the flagellum 

 of B. termo 0*00000488526, which, expressed in vulgar fractions, 

 is equivalent to 1 /204700th of an inch nearly; that is to say. 

 within a wholly inappreciable quantity." 



These classical measurements of the diameter of the B. termo 

 flagellum are of the greatest importance, for by their means the 

 accuracy of the extinction method is demonstrated, which in turn 

 serves to confirm the exactness of the late Dr. Dallinger's results. 

 Assuming that a W.A. of 8 was employed, the necessary anti- 

 point correction by Mr. Nelson's amended table is 0*000005 13th in., 

 which,addedtoDr.Dallinger'smean,makes0*00001001(l/99,900th) 

 in. for the true diameter of the flagellum, as against 0*00000891 



* Journ. Q.M.C., Ser. 2, Vol. XI. PI. 22. 

 f Ibid., 1878, pp. 109-75. 



