PKOCEEDINGS OF THE SOCIETY. 239 



or perhaps the best, was at least effective, and it had the advantage of 

 simpHcity. The one part of the microscope which was at the bottom of 

 the trouble in centration, either for dark-ground illumination or for 

 direct illumination, was the mirror. He had made a board on which 

 there was an acetylene lamp, a condensing lens, and a fixed mirror which 

 was inclined at the proper angle to project a beam of light which was 

 vertical to the board. This he thought would be a valuable method 

 for microscopic work, and would do away with many of those errors of 

 centration which arise. Mr. Barnard then gave the following direc- 

 tions : — Remove the sub-stage mirror from the microscope, and place 

 the microscope centrally over the mirror on the board (unless set within 

 narrow limits of accuracy nothing will be seen). To adjust the instru- 

 ment remove the dark-ground illuminator from the microscope, put in 

 a low-power ocular only, and put the source of light at such a position 

 in relation to the condensing lens — which must be used between the 

 mirror and the source of light— that a rough image of the illuminant 

 is thrown on to the centre of the mirror. Then project this light up 

 through the microscope tube, there being nothing intervening between 

 the mirror and the ocular, and throw on to the ceiling a disc of Hght, 

 and that disc is a perfect indication of centration. If there is any 

 want of centration, some reflection may be seen from either the inside 

 of the tube of the microscope or the inside of the ocular, or from the 

 sub-stage fitting if there is considerable lack of concentration. When 

 centrality is secured do not touch the light, the condensing lens or the 

 mirror in any way. Put the objective on, put on to the stage the 

 object to be observed, having previously put in the dark-ground 

 illuminator, with either oil or glycerin to secure immersion. Next get 

 the object into view with a low-power objective, focus up the image of 

 the radiant, getting as sharp and as small an image of it as possible. 

 When viewed, if not at the centre of the field, bring to the centre by 

 means of the sub-stage centring-screws. Then replace the low-power 

 ocular by the higher power, x 12 or x 18, and all should be in order. 

 Finally, slides and cover-glasses should be carefully calibrated, and 

 cleanliness of these two essential accessories was of the greatest 

 importance. 



Messrs. Akehurst, Heron-Allen, Sheppard, Scourfield and Dr. Rudd 

 Leeson took part in the discussion that followed. 



Colonel Harrison, M.D., on the invitation of the President, dealt 

 briefly with the difference between various Spirochaetes. In distin- 

 guishing S. pallida from other varieties, the points he laid stress upon 

 were : first, the extreme fineness of the pallida ; secondly, its pallidity. 

 If the focus was shifted, even a little, other varieties assumed a rusty 

 hue, whereas the S. pallida retained its delicate pallor. S. perfringens, 

 too, was much more dazzling than S. pallida. Hence the illumination 

 which sufficed fairly well for other Spirocha^tes was not enough for 

 >S'. pallida. Another feature was the great flexibility of S. pallida, 



