440 PROCEEDINGS OF THE SOCIETY. 



A paper by Mr. E. Moore Mumford on " The Higher Bacteria 

 (Sjjhserotilns) " was read by Dr. Shilhngton Scales. 



The President said that he had had the opportunity of looking- over 

 Mr. Momford's paper on " The Higher Bacteria," and he had found it 

 to be one of very particular interest from several points of view. Up 

 to tlie present no very definite effort had lieen made to distinguish the 

 various species of Sphserotihis. He did not know that the author had 

 succeeded in establishing any really distinctive features, but he had 

 made out a certain number of differences of specific value. Another 

 point of interest was that Mr. Mumford's method seemed to promise 

 better results in this particular branch of study, because in it one might 

 have a means of distinguishing the amount of crude organic matter 

 contained in water ; indeed, if it were true that these Sphserotili had 

 a power of selecting very minute quantities of sewage, it might be of 

 great value. Here was a line of investigation which, if successful, would 

 provide an additional weapon in our bacteriological armamentarium. 



A vote of thanks was proposed to Mr. Mumford, and unauimously 

 carried. 



Mr. Hamilton Hartridge, M.A., F.R.M.S., read the first of his two 

 papers on " The Measurement of Working Aperture." 



In response to the Chairman's request for remarks on Mr. Hartridge's 

 paper, Dr. Shillington Scales read the following letter from Mr. Crordon, 

 offering criticism of Mr. Hartridge's methods. 



113 Broadhuest Gabdens, 



West Hampstead, N.W. 



June 13, 1918. 



Dear Sirs, 



I have to acknowledge and to thank you for copies of the proofs 

 of two papers to be read before the Royal Microscopical Society on 

 Wednesday next by Mr. Hartridge. These I have read with very great 

 interest, and perhaps you will allow me through you, as I am not 

 personally acquainted with Mr. Hartridge, to call his attention to one 

 or two points that may, I think, be of interest to him. 



With respect to the measurement of the working aperture Mr. FTart- 

 ridge is adopting the same method which I described to the Society 

 in 1907, in connexion with my top-stop apparatus. Mr. Hartridge 

 has recourse to an apertometer plate for the purpose of calibrating his 

 Ramsden disk. I adopted what appeared to me to be the simpler 

 method of taking the equivalent focal length in air of the Microscope 

 as a whole, and drawing a circle with this magnitude for radius. The 

 circle so detei"mined, placed in the Ramsden disk, affords a direct 

 measure of N.A. = 1. This method of calibrating the Ramsden disk 

 ap])eared to me to be quite satisfactory, but it may be that it is less 

 exact than Mr. Hartridge's method. If so it would, I think, be of 

 interest if he would let microscopists know by how much the aperto- 

 meter method excels the other method in precision. . . . 



J. W. Gordon. 



