PROCEEDINGS OF SOCIETIES. 233 



aperture a little larger than the field of view with the lowest eye- 

 piece. This could be brought into contact with the covering glass of 

 an object, when it excluded all extraneous rays. The effect was 

 almost like magic ; the whole of the false light was got rid of, the 

 silvery or milky appearance of diatoms, usually present when viewed 

 by large-angled objectives, disappeared, and their markings and 

 outlines were beautifully black and sharply defined. He had not 

 yet measured the angle, but thought that it could not be greatly 

 reduced by the cap, for, with the light thrown by an achromatic 

 prism at the greatest obliquity, so that the field was half dark and 

 half light, Grammatophora subtilissima was well resolved. He did 

 not, however, care so much for the alteration for diatom work as for 

 getting rid of false light in general observation. The subject was one 

 of great importance, and Mr. Wenham's remarks were most valuable. 



Mr. Slack said it seemed that it was rather a mistake to give such 

 a very superfluous quantity of front lens. He had an ^-inch objective 

 made by Thomas Eoss, and if he allowed the condenser to throw light 

 upon the whole of the front lens, anyone would be inclined to think, 

 on looking through it, that the object-glass was not worth a penny, 

 but when he allowed the light to strike only upon the necessary part 

 the effect was very fine. 



Mr. Wenham then drew a diagram showing how the outer portions 

 of the front lens might with advantage be ground away, and ex- 

 pressed his oj)inion that this would in every case improve the light. 

 The marginal parts were perfectly useless, and they could be very 

 easily ground off now that the single front had become the universal 

 form. 



Mr. Stephenson mentioned that some years ago he had a j-inch 

 objective by Ross which gave him a good deal of trouble from the 

 same cause, but Mr. Hewitt made a cap for it which i^erfectly cured 

 it. He had listened with much interest to Mr. Wenham's remarks, 

 and should like to ask how it was that the thickness of the foil was 

 prevented from reducing the aperture of the lens ? 



Mr. Wenham said that the thickness of the foil was extremely 

 slight, not more than yoVo i^ich, and it was brought exactly into the 

 focus of the lens, and there it did not cut off* any apertui-e. If it was 

 moved it would show at once, by its edges encroaching upon the field, 

 when any of the oblique rays were being cut off. 



Mr. Stephenson inquired if it were placed above the level of the 

 glass, or below ? 



Mr. Wenham said it was above the level. With a ^V inch objective 

 the separation was about the yi^ inch, and it was brought quite to a 

 knife-edge. 



Mr. Slack suggested the deposition of a film instead of using foil. 

 It was quite clear to him that Mr. Wenham's observations would lead 

 to a considerable modification of all their ideas of the need of extreme 

 angles if they were to knock off' 20, 30, 40, or 60 per cent, of the 

 usually-received measurement. 



Mr. Frank Crisp inquired whether the higher powers were not 

 much more reduced than the lower ones ? 



Mr. Wenham said that the higher the power the greater was the 



