PEOCEEDINGS OF THE SOCIETY. 733 



The Chairman also called attention to an exhibition of the palates 

 of Mollusca from the Society's collection, which were shown under a 

 number of Microscopes lent for the occasion by Messrs. Watson & Sons, 

 to whom the thanks of the Meeting were unanimously voted for the use 

 of these instruments. 



New Fellows. — The following were elected Ordinary Fellows : — 

 Mr. William Joseph Dibdin, Rev. William Earl, Messrs. Charles E. M. 

 Fischer, Alfred G. E. Foster, John Ross MacKenzie, and Cecil Price- 

 Jones. 



The Following Objects, Instruments, &c. were exhibited : — 



The Society : — An Old Microscope by Dollond, after John Cuff ; an 

 Old Microscope by Banks, after " Jones's Most Improved Compound 

 Microscope " ; a Selection of ten Slides of Palates of Mollusca from the 

 Saunders Collection. 



Mr. A. E. Conrady :— The following Experiments in illustration of 

 his paper. I. Diffraction Spectra with Curved Wave-fronts. The 

 condenser may be racked up or down as far as it will go, causing a 

 variation incurvature of the waves passing through the grating from the 

 maximum possible to practically plane waves (considering the smallness 

 of the object) without amy change being noticeable in the diffraction 

 spectra. II. Diffraction Spectra with Critical Light-Opening and Closing 

 of the Condenser-Iris cause the diffraction spectra to expand and con- 

 tract in uniform circles, always retaining their distance from centre to 

 centre, just as would be the case with diffused light. III. Dr. Johnstone- 

 Stoney's Experiment. An apochromatic objective (Zeiss 16 mm., 

 •30N.A.) is used as a condenser, carefully adjusted and focussed, the 

 " Diffractions-platte " being placed upside down. According to the usual 

 interpretation of the undulatory theory, diffraction spectra should be 

 impossible in this case. It will be seen, as a matter of fact, that they 

 are there, precisely like those in Experiment I. 



Mr. J. W. Gordon : — Experiment referred to in Mr. Conrady's 

 paper, to show the effect of shortening the radius of curvature of the 

 diffracted wave-front where it passes the grating. 



Mr. Hugh C. Ross :— An Electric Warm Stage. 



Owing to the lateness of the hour Mr. Conrady was precluded from 

 replying, but has kindly favoured us with the following rejoinder to 

 Mr. Gordon's remarks : — 



Mr. Gordon sent me a signed copy of the manuscript of his note a 

 day or two before the meeting at which my paper was read. 



In this note Mr. Gordon endeavours, inter alia, to show that I 

 made a miscalculation in the treatment of a certain integral by which 

 I prove an important new theorem applying to gratings. It was imme- 

 diately apparent to me that Mr. Gordon had misunderstood or over- 

 looked the carefully explained meaning of the two angles entering into 



