246 



SUMMAItY of cuim;knt ukska KGHES 1,'KLATING to 



pressure on the object measured. The larger size measures to approxi- 

 mately 5^j of a millimetre. 



Fig. 55. 



Quekett Microscopical Club. — The 445th Ordinary Meeting of 

 the Club was held on January 17, the President, Dr. E. J. Spitta, 

 F.R.A.S. F.R.M.S., in the Chair. Owing to the unfortunate absence 

 through illness of the authors, neither of the two papers announced 

 were read. Messrs. Baker exhibited with the lantern a number of 

 slides, mostly of pond life. Mr. E. Large, using the projection 

 polariscope, exhibited some very interesting and beautiful sections of 

 selenite crystals, also some photomicrographs of twinned crystals. 



At the 446th Ordinary Meeting, which was also the 42nd Annual 

 General Meeting, Professor E. A. Minchin, M.A. (Oxon.), was elected 

 President. The usual reports, which were very satisfactory, were 

 presented by the Committee, Treasurer, Librarian, and Curator. 

 Dr. E. J. Spitta, F.R.A.S. F.R.M.S., the retiring President, delivered 

 the Annual Presidential Address, taking for his subject " The Photo- 

 graphy of Very Translucent Diatoms at High Magnifications." Refer- 

 ence was made to the difficulty of obtaining contrast between the 

 object and the background, and this being due to the nearness of the 

 index of refraction of the mounting medium to that of the silex of the 

 diatom (l - 43) (Canada balsam is 1'52), it was advised that, if possible, 

 diatoms to be photographed under high powers should be mounted in 

 realgar, the "index of visibility " of which is 121, that of Canada balsam 

 being only 9. The "fog" seen round dot markings was stated to be 

 caused by the fact that no lens, or combination of lenses, can represent 

 the image of a point as another point, but such must be shown as a disk 

 of more or less sensible diameter. This " fog " is got rid of in the 

 following manner : — A negative is made on a fast plate, and is developed 

 preferably with hydrokinone to obtain maximum contrast. A positive 



