386 PKOCEEDINGS OF THE SOCIETY. 



remarks as to polarisation, the passaj^re of light through a tiny aperture 

 caused it to be diffracted, and diffracted light was polarised. 



Mr. J. W. Gordon said with regard to total blackness, this was not a 

 question of Stokes' layer, but was certainly due to something which had 

 intervened between the cover-glass and the specimen (in illustration of 

 which a diagram was drawn on the board). If it were a mere question 

 of Stokes' layer, reflection would occur from the back of the specimen, 

 which in that case would be uncovered. To produce the total blackness 

 described by Mr. Nelson, the homogeneous medium must fill the entire 

 space between the cover-glass and the slip. Stokes' layer, therefore, 

 could not afford any explanation whatever. With regard to Mr. 

 Conrady's second hypothesis, he did not understand Mr. Nelson to say 

 that the light was polarised, but that it was depolarised. He (Mr. 

 Gordon) was not aware that diffracted light was polarised when diffracted 

 by a transparent diffractor as distinguished from a reflector, but in any 

 case it seemed impossible that a mere aperture should serve as a de- 

 polarising agent.* 



On the motion of the Chairman, a vote of thanks to Mr. Nelson 

 for his paper was unanimously passed. 



A paper by Dr. Antonio Rodella, " On the Root Bacteria of Pulse," 

 was read by Dr. Hebb, and the thanks of the Meeting were voted to the 

 author. 



Dr. Ettles exhibited and described the Ettles-Curties Ophthalmometer 

 and a Corneal Microscope. 



A demonstration of the application of the instrument was given at 

 the close of the Meeting, upon a patient with pronounced astigmatic 

 vision, who presented himself for the purpose of the experiment. 



The thanks of the Society were unanimously voted to Dr. Ettles fur 

 exhibiting the instrument, and for his subsequent demonstration of its 

 action. 



.It was announced that at the next Meeting of the Society (on 

 May 15) there would be an exhibition of Pond-life. 



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



Mr. J. E. Barnard : — Illumination l^y means of a Mercury Yapour 

 Electric Lamp, and different colour screens, under three Microscopes. 



Mr. A. Earland : — Exhibition of slides of Arenaceous Foraminifera, 

 as follows :— 



1. Arenaceous Foraminifera from various parts, and from shore sands 

 to 2500 fathoms. Selected to show the great variety of materials used 



* Mr. Gordon desires to add that since the Meeting he has examined with a 

 Nicol's prism both the spectrum of a transparent grating and the diffracted light 

 from a Podiira scale. In neither case could he find any visible trace of polarised 

 light. He suggests that j\Ir. Conrady may have been misled by observing the 

 polarisation which probably does occur in the reflected spectra from a metallic 

 grating. 



