348 PROCEEDINGS OF THE SOCIETY. 



was exceedingly valuable in the case of large-angled objectives. He had 

 no valve of Amiiliiijleura pellucida which he had not been able to 

 resolve although he had been often told by ojiticians that some of his 

 slides were of no value, being quite "washed out"; as for Surirella 

 gemma, the whole valve was seen to be covered with knobs. Mr. 

 Morehouse, in the extract quoted in the last number of the Journal 

 (p. 194), pointed out that the vertical illuminator would only work well 

 with large aperture lenses, and it would be found that it was only 

 with very large angles, exceeding 180°, that it acted effectively. 



Mr. Curties asked what Mr. Stephenson considered the best form 

 of illuminator. 



Dr. Edmunds said he would be glad to know whether Mr. 

 Stephenson had compared the reflecting prism, the disk of thin 

 glass, and the opaque steel mirror as practical tools ? Would it be 

 best to work the illuminator from the side of the Microscope tube, or 

 in the optic axis, and at what point behind the objective would the 

 reflector work best on the object, and do the least damage to the 

 image received by the eye-piece? 



Mr. Stephenson said that the apparatus he had used was the one 

 with a parallel plate of glass. In one respect a small prism was no 

 doubt better, because with the plate of glass light was received from 

 both surfaces, which tended to confuse the image. The prism was 

 certainly better than the steel disk, and it was essential that it should 

 be placed at the side of the tube. Of course just so far as the prism 

 projected over the edge of the objective, the aperture of the glass 

 would be diminished. Dr. Carpenter gave the preference to the thin 

 glass disk over the fixed parallel plate, both on accoimt of its 

 superior reflecting power, and the ease with which it could be set at 

 any inclination. 



Mr. Crisp said that Mr. Stephenson's demonstration of the excess 

 of aperture over 180° was the most interesting that had yet been 

 suggested on that subject. With regard to slides of Amphipleura 

 being washed out, he had been frequently assured when objectives 

 were being tried on his own slides, that the slides were " not those of 

 the Amphipleura which had markings, but a variety which had no 

 markings." It should be mentioned that Mr. Adolf Schulze had 

 early last year discovered the power of the vertical illuminator, when 

 used with oil-immersion objectives, to resolve AmpMpleura. He 

 unfortunately delayed the publication of the method through an 

 accident. 



The President being obliged to leave, the chair was taken by Dr. 

 Braithwaite, V.P. 



Mr. Crisp brought forward the resolution of which he gave notice 

 at the last meeting, as to a standard unit of micrometry. He said 

 that he had little to add to what he then stated. His motive in bringing 

 the resolution forward was, 1st, that they as the oldest Microscopical 

 Society in existence, should express an opinion one way or the other 

 on a subject which was considerably agitating their fellow workers in 

 America, and 2nd, his conviction that it would be a grievous error for 



