PROCEEDINGS OF THE SOCIETY. 131 



for his donation, to Sir F. Crisp for having forwarded it, and to Mr. 

 Rousselet for his description of it. 



Mr. C. Beck exhibited and described a new method of showing 

 bacteria by dark-ground illumination, being a modification of the para- 

 bolic illuminator, used in conjunction with a Nernst lamp and mono- 

 chromatic blue light. The new apparatus was devised by him to obviate 

 the inconvenience caused by the oil running down the reflecting surfaces 

 of the illuminator and allow a perfect focusing adjustment for the light. 

 It was pointed out that when bacteria were shown on a bright ground 

 they appeared only like black lines, whilst when seen on a dark ground 

 they were rendered far more distinct, although their appearance varied 

 somewhat according to what parts reflected light most brilliantly. In 

 general the appearance seen was that of a brilliantly illuminated envelope 

 and bright nuclei ; if there was a twist in the specimen under observation 

 they would get nodes of light at the twists, whilst in other cases an 

 extremely brilliant circular patch would be seen in one place. In 

 employing this method it was important to have a perfectly clean slide 

 only containing the creatures it was desired to examine, since every 

 particle in the field would be strongly reflective and a mass of brilliant 

 material in the background prevented the examination of objects in 

 the foreground. He thought this method of illumination was well 

 worth considering, as being much more likely to give a correct idea of 

 what was being seen, than if the ordinary method was employed. The 

 construction and action of the parabola was explained by means of 

 diagrams on the board. 



The Chairman said there could be no question as to the difficulty of 

 getting photographs of unstained bacteria seen in the ordinary way, and 

 the process described by Mr. Beck certainly seemed to be worth atten- 

 tion. The only difficulty which occurred to him in connection with the 

 matter was that it was limited to a numerical aperture of 1, but he 

 thought their hearty congratulations were due to Mr. Beck for what he 

 had accomplished. Everything new was of value, for even if it was not 

 apparent at the moment it might be in the future, when it was most 

 convenient to find a piece of apparatus, just what you wanted, read) 

 to hand. 



Mr. Beck said the angular aperture was limited by the fact that in 

 looking at bacteria they were seen in water which had a refractive index 

 of 1 • 38. The actual angle of the illuminator was from 1-1 to 1 • 5, but 

 this was cut down by the water. If seen in oil the angle would, of 

 course, be higher. 



The thanks of the Society were voted to Mr. Beck for his com- 

 munication. 



Mr. J. W. Ogilvy exhibited and described a new Microscope by Leitz ; 

 diagrams showing the mechanism of the fine-adjustment were placed 

 upon the table. 



The Annual Report of the Society for 1 907 was then read by Dr. Hebb. 



