QUEKETT MICROSCOPICAL CLUB. 363 



per cent, alcohol. Only the skeleton is stained, but the muscular 

 tissue may be stained by adding to the alizarin a weak solution of 

 Bismarck brown. 



A paper by Mr. C. D, Soar on a species of Hydracarina found at 

 Bear Island by the Oxford University Expedition to Spitsbergen, 

 1921, was taken as read, and the Secretary read two notes by Mr. 

 E. M. Nelson. In the first note Mr. Nelson said that in setting 

 up a dark-ground illumination with a bull's-eye (say for pond 

 life) it was usual to place the flame at the principal focus of the 

 bull's-eye, so that parallel rays might fall on the back lens of the 

 substage condenser. Therefore the image in the object-plane 

 would be the bright disk of the bull's-eye. If the bull's-eye is 

 placed a little farther from the edge of the flame, so that the 

 image of the edge of the flame falls upon the back lens of the 

 substage condenser, then the image on the object-plane will 

 be a magnified image of this flame, and when that is cut out 

 by a stop of proper size, the brilliancy of the illumination will 

 be considerably increased. The sharpest, but not the brightest, 

 image is obtained when a bull's-eye is dispensed with, and the 

 flame image focused upon the object-plane by the substage 

 condenser. A little difference in the size of the stop makes a 

 good deal of difference in the brightness of the illumination, 

 especially if the power of the substage condenser is high. 



Mr. Nelson said that for transmitted light, especially with 

 high powers, the screen he had used for many years was one 

 composed of three glasses, to which a fourth in special instances 

 was added. This he preferred to all others. In the Wratten 

 scale it comes between the Nos. 44 and 45, which are both good 

 screens for visual work upon diatoms. With bacteria the con- 

 ditions are quite dissimilar, so he used entirely different glasses, 

 sometimes as many as five. These, however, can be matched 

 by Wratten' s 58 + 65a, which he thought gave perhaps a 

 better result than the glasses. Mr. Nelson's other note described 

 how to distinguish tobacco leaves from those used as adulterants, 

 of which there are about forty. The tobacco is soaked in hot 

 water for a quarter of an hour and then a fragment is examined 

 in a compressor with a drop of water. If glandular hairs three 

 or four cells long are present, having at the tip an oval cluster 

 of some six cells filled with a yellow substance, the plant is tobacco. 

 The thanks of the Club were accorded to the authors of the above 



