PEOCEEDINGS OF THE SOCIETY. 571 



Mr. J. A. Hill : — Slides of Bacteria, in illustration of his communi- 

 cation. 



Dr. H. Siedentopf : — Three Microscopes, fitted with special illumi- 

 nating apparatus, showing methods of rendering visible ultra-microscopic 

 particles in gold ruby glasses, and in colloidal solutions, also a method 

 suggested for rendering visible ultra-microscopic bacteria. 



The following, with other specimens of ruby glass, lent by Dr. 

 Zsigmondy, of Jena : — Copper ruby glass, containing 2 p.c. of copper, 

 originally transparent, 150 times heated and cooled ; this process causing 

 the glass to become perfectly opaque. A gold ruby glass that had been 

 subjected to a high temperature, which had caused small particles of gold 

 to unite and form larger particles, ultra-microscopical objects being thus 

 cemented into microscopical objects. The following gold solutions 

 prepared and lent by Dr. Zsigmondy (all the solutions contained 

 0-005 p.c. of gold) :— 



2. 



4. Cloudy gold solution. „ solution, red- violet. 



„ particles, green and yellow, 

 medium size. 

 Particles showing vivid oscillations. 



5. Colloidal solution of gold. Particles smaller than 10 //,//. = 



tooVWo mm - Particles show- 

 ing very vivid oscillations. 



6. „ „ Particles so small that they cannot 



be rendered visible. 

 A water-immersion objective D*, f, 4*4 mm., working distance 

 1*5 mm., N.A. 0'75. 



In connection with Lord Rayleigh's paper, Mr. Gordon writes as 

 follows under date June 18. 



" May I ask leave to point out to the readers of the Journal the 

 great practical consequence of the result which Lord Rayleigh has now 

 established. 



i In a sense, which I propose to illustrate by a familiar instance, the 

 visibility of a dark bar of finite dimensions on a bright uniformly 

 illuminated field is the ultimate condition of resolving power in the 

 Microscope, and a much more proper test than the diffraction grating 

 limit. This will appear from the following example. Under an objec- 

 tive of moderate power the diatom known as Triceratium Favus appears 

 as a nearly uniform bright field divided up into hexagons by very fine 

 boundary lines — " dark bars " in the sense of Lord Rayleigh's paper. 

 What the dimensions of these bright areas and limiting lines precisely 

 are I do not know, but I suppose that if I took the bright hexagonal 



