238 PROCEEDINGS OF THE SOCIETY. 



The President (Mr. J. E. Barnard) prefaced his remarks on Dark 

 ground Illumination by stating that in this method the object had to be 

 illuminated in such a way that no direct light entered the microscope. 

 This could be achieved by placing a " stop " at the back of an ordinary 

 achromatic or any suitable condenser, or, preferably, by using either 

 a paraboidal or a spherical surface-reflecting condenser made specially 

 for the purpose. He did not propose to discuss the relative merits of 

 particular dark-ground illuminators ; each had its good points, and most 

 of them were capable of producing satisfactory dark-ground effects. 

 (At the close of the meeting several of the best-known illuminators 

 were shown, and the particular type of beam projected by each was 

 demonstrated.) 



He then pointed out that success or failure depended upon :■' — 



1. Selection of illumination. 



2. Exact centrality of all the optical parts of the microscope. 

 The selection of the illuminant was a difficult problem. The 



tendency was to employ a source of light of too great intensity. An 

 ordinary electric filament lamp gave light of large dimension in one 

 direction and small in the other. The result was to fill the condenser 

 in one direction and iiot in the other, so that with such objects as 

 Spirocha?tes perfect illumination of the convolutions throughout the 

 length of the organisms was lacking. An arc lamp could be employed, but 

 with its use many inconveniences were experienced. The background 

 was not so black as it should be ; the image was not so well shown, and, 

 especially when dealing with blood-plasma, minute particles, usually 

 described as "ultra-microscopic," become annoyingly obvious. The 

 ideal source of light for this method was the " Pointolite " lamp of the 

 Ediswan Co. It consisted of a 4-mm. tungsten sphere, and when incan- 

 descent its image could be projected into the object-plane and perfect 

 uniform illumination secured. 



Another point which had an important bearing on this question of 

 the intensity of the source of light was this : If one took an objective, 

 such as a 4-mm. Apo. N.A. ' 95, and illuminated the object by means of 

 an arc light, one was unable to get satisfactory dark-ground illumina- 

 tion with four condensers out of five. With some it Avas possible, but 

 only when the obliquity of the light was at its maximum. Take a 

 weaker source of light — he had one which he had been using for the last 

 day or two — and it became possible to use a 0*95 N.A. objective with 

 any condenser, and, while the ground was not black, it was dark enough 

 to enable Spirochseta pallida to be seen with all its characteristic features. 

 It was true a fair amount of direct light came through, as well as an 

 obHque beam which went to form the dark-ground image. He had 

 been criticized for using only a dry lens for dark-ground work, but 

 Professor Conrady had now justified the method. On theoretical grounds, 

 it was clear that an objective of 0"65 N.A. was doing all that could be 

 done, although it might, under certain circumstances, be advisable to 

 use an objective of higher numerical aperture. Therefore the use of a 

 very powerful source of light was to be avoided. 



The next point was that of centration. The matter was really a 

 very simple one. The method he advocated, though not the only one 



