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



extinction of the image of a minute point by reducing the W.A. 

 to 0'165. The size of the point was measured by a wide-angled 

 oil-immersion, and a W.A. of 0*9, and was found to be apparently 

 1/50, 050th inch. From this we have 



6-6961X-165 = 50,050. 

 And 



:0-000003663. 



li-6961\-9 



Employing this as a provisional correction, we find the size of the 

 point to be 1/4 2,396th in. Again, using this measurement, we 

 obtain a new numerical coefficient, viz. 5*6587, and finally find 

 the size of the point 1/40, 875th in., and the coefficient 5*4686 as 

 stated above. In this calculation A is the reciprocal of the wave- 

 length, or the number of waves per inch, given at the head of 

 each column in the table." In Mr. Nelson's subsequent paper, 

 " The Influence of the Antipoint on the Microscopical Image 

 shown graphically," the data will be found for the slightly 

 amended table given therein. 



Shortly after the publication of Mr. Nelson's papers on this 

 interesting subject, Dr. Coles kindly sent me a well-stained 

 balsamed slide of the putrefactive microbe B. termo. On this I 

 was able to find a distinctly flagellated specimen suitable for 

 measurement by the extinction method. The flagellum could be 

 plainly seen with an apochromatic l/6th of 0*98 N.A. used with 

 a full cone and screen, and it became invisible when the N.A. 

 was gradually cut down to 0*42 by means of an iris diaphragm 

 over the top lens of the objective, thus making the diameter of 

 the flagellum 0*00000S91 (1/1 1 2,200th) in., or 0*226 fx. 



Afterwards a balsamed-stained, flagellated specimen of the 

 tubercle bacillus was found. This was more difficult to see, and 

 the flagellum was thought to be much finer than that of the 

 B. termo. A 1/8 tli apochromat of 1*4 N.A. was employed to 

 measure this. When the N.A. was cut down to the vanishing 

 point and tested with the Abbe apertometer, it was found to bv 

 exactly 0*42, thus making the diameter of the tubercle bacillus 

 flagellum precisely equal to that of the B. termo. It may here 

 be mentioned that the existence of the tubercle bacillus flagellum, 

 discovered by Mr. Nelson, has been denied. It has, however, 



