PROCEEDINGS OF THE SOCIETY. 271 



could be seen at all in non-motile organisms. He remembered on one 

 occasion being invited to see the flagella of tubercle bacilli, but was 

 quite unable to do so, though others who were present recognised them 

 easily. All these points were quite caviare to him, but then he did not 

 profess to be a critical Microscopist. 



Mr. F. Shillington Scales thought that some of these measurements 

 were of such a nature and made such exaggerated cla'ms to minute 

 accuracy of measurement as rather to bring the subject of micrometry 

 into discredit amongst practical workers in laboratories. Their old friend 

 the tubercle bacillus had been a source of considerable controversy, and 

 the very existence of its flagella was denied by bacteriologists, yet the 

 subject was constantly reverted to with the same result, and here claims 

 were made as to actual measurements of the flagella. He thought it 

 was a pity that so much attention should be given to the subject. 



Mr. Rheinberg thought the extinction method of measurement was 

 interesting, but had Mr. Merlin been present he would have liked to ask 

 him whether precautions were taken as to the strength of the illumina- 

 tion, which he thought was a factor of importance for any comparative 

 measurements of this description. The apparent size of a flagellum was 

 greatly determined by the strength of the illumination, as this deter- 

 mined the apparent size of the diffraction disks which overlapped the 

 flagellum, and the degree of opacity of the flagellum would also affect 

 the result. 



Mr. Conrad Beck, while expressing great interest in the extinction 

 method of measurement, did not think it could be used except by the 

 most expert microscopist who had perfect command of the illumination, 

 and even then did not consider the results obtained could be accepted 

 without complete confirmation from some other method, as too little 

 was known as to the limits of resolution under different conditions of 

 illumination. The limits of resolution of double stars had even been 

 called in question of late, although Mr. Nelson's work on this subject 

 had not been confirmed by Professor Porter. 



Mr. Conrady said the great objection to the extinction method of 

 measurement was that it required a knowledge of the degree of opacity 

 of the object. It was an easy matter to compute the width required 

 for an absolutely opaque object to be just visible with a given numerical 

 aperture, but a semi-transparent object would have to be broader, nearly 

 in proportion to the light which it transmitted, in order to become 

 visible under similar conditions. For this reason such a method seemed 

 to him illusory. 



Mr. J. E. Barnard said that, looked at from a practical point of view, 

 microbes, even of the same species in any one culture, varied very con- 

 siderably in size, and it might, therefore, be inferred that the flagella 

 would vary also ; the actual benefit of exact determinations in such 

 cases therefore seems doubtful. As to the point raised by Mr. Conrady, 

 by the silver deposition method of staining an absolutely opaque object, 

 could be produced by this means, although, of course, the thickness of 

 the silver film was unknown. 



Mr. Conrady begged leave to say that the silver would have to be 

 deposited in a very appreciable thickness in order to be quite opaque. 



