PROCEEDINGS OF THE SOCIETY. 337 



dark-ground illuiniiiation was obtained. Tlie experiment as arranged 

 demonstrated that diffraction played as important a part in dark-ground 

 illumination as it did in the formation of microscopical images by direct 

 illumination. 



He had also set up two Microscopes to demonstrate other methods 

 of dark-ground illumination. One showed gamboge particles, the 

 dimensions of which was of the order of • 1 micron ; and the (^tlier 

 bacteria illuminated by means of an 8 mm. apochromatic objective. 



Mr. Blood asked whether Mr. Barnard, in his first exl libit, suggested 

 that all dark-ground illumination was due entirely to diffraction effect. 

 Also how Mr. Barnard arrived at the size of the gamboge particles, and 

 why he assnmed they were spherical. 



Mr. Barnard replied that he maintained that in dark-ground illumi- 

 nation the diffraction effects were of the utmost importance. It had, 

 perhaps, been sufficiently realized that this was the case. It was often 

 assumed that reflection and refraction were the chief, if not the only, 

 points of importance in the formation of the image, but clearly the 

 results of diffraction must be regarded as of primary importance. 



Mr. Spitta said that he knew of no theory asserting that gam1)oge 

 particles, or the visible ultimate-particles of any solid, must be — ex 

 '/^^r«.s•s /Me— spheres, although he believed investigation had proved this 

 form as correct with gases. 



Mr. Barnard replied that his authority was Jean Perrin, who dealt 

 with this point in his book, Brownian Movement and Molecular Reality, 

 his proof being both mathematical and the result of observation. 



The thanks of the Society were cordially voted to the Members of 

 the Biological Section for their interesting exl libit, and especially to 

 Messrs. Scourfield and Barnard for their demonstrations. 



Mr. James Murray made a communication (No. II.) on " South 

 American Rotifera " (which will appear in extenso in the Journal), and 

 appealed to the Council to permit him to present the whole of the series 

 at once to the Society, as he was about to leave England on a long voyage 

 in the Northern Canadian Arctic Regions, when he would be away some 

 years. 



Permission was given to Mr. Murray to do this. 



Dr. Shilhngton Scales said that the Society always welcomed Mr. 

 Murray's papers, and was proud to publish such w^ork. 



The Chairman wished Mr. Murray hon voyage, and expressed the 

 hope that in the near future he would be back with another series of 

 papers. 



A very hearty vote of thanks to Mr. Murray for his communication 

 was carried wdth acclamation. 



It was announced that the next Meeting would be held on Wednesday; 

 May 21, when there would be held the annual exhibition of Pond-Life. 



