PROCEEDINGS OF THE SOCIETY. 75:> 



still subtends an angle many, very many times, as great as that sub- 

 tended by a star. The fact is that in such a case what we really see is 

 the cloud apparently divided into two parts by the wire, which is not 

 itself directly visible. Thus the problem of seeing a telegraph wire 

 against the. background of a bright cloud is truly a problem of resolu- 

 tion, for, accurately stated, it must be formulated thus : ' What is the 

 smallest angular distance asunder at which we can see the two parts into 

 which the telegraph wire appears to divide the cloud ? ' We find, in 

 accordance with this principle, that beyond a certain very feeble degree 

 of illumination, increase of brightness does not increase the visibility of 

 the small dark object. The stem of a weather-cock strongly visible 

 against a bright sky will disappear from view entirely if the disk of the 

 sun comes behind it, and our failure to see it under those conditions is 

 simply a failure to obtain a resolved image of the sun's disk. Interpose 

 a smoked glass in front of your eye and you will see the stem of the 

 weather-cock perfectly outlined against the disk of the sun. The same 

 considerations apply to stained bacteria, or other small dark objects, seen 

 in a bright field, which can be seen only by means of an instrument 

 which yields a well resolved image of the field." 



A vote of thanks to Mr. Gordon for his paper, and to those who had 

 taken part in the discussion, was unanimously passed. 



Dr. Hebb read a letter which had been received from the Secretary 

 of the Optical Society. 



The Chairman reminded the Fellows that at their next Meeting it 

 would be necessary to nominate Officers and Council for the ensuing 

 year, and also to appoint two Auditors of the Society's accounts for the 

 year 1906. 



The following Instruments, Objects, etc., were exhibited : — 



The Society : — An Old Microscope, by Dollond, presented by Mr. 

 Charles Lees Curties ; and an Old Microscope, by Culpeper, presented 

 by Mr. Wynne E. Baxter. 



Mr. Conrad Beck : — Optical Bench, for illumination with either 

 ordinary or monochromatic light, arranged to show experimentally that 

 AmphipUura pellucida could be resolved by the green light, while it 

 could not be resolved by the yellow light. 



Mr. J. W. Gordon : — Specimen of Staphylococcus shown with the 

 aid of a top stop, being the original specimen used for the figure that 

 will appear in the Journal in illustration of his paper ; Photomicrographs 

 and Lantern Slides shown on the screen in illustration of his paper. 



Dr. Hebb :— Doulton's White Porcelain Filter ; Small Filter Bottle, 

 for filtering micro-mounting fluids, devised by Mr. H. Taverner. 



