270 PROCEEDINGS OF THE 



valve surface. An examination of fractures under critical con- 

 ditions confirmed Mr. Merlin's interpretation of tlie secondaries as 

 perforations. Increasing the illuminating cone when the " white 

 dots " are in focus causes them to disappear, while the same 

 experiment with the black dots merely reduces the contrast and 

 they remain in focus. Mr. Merlin said that most of the observa- 

 tional facts mentioned were due to Mr. Nelson ; he claimed no 

 originality for any of them. 



Commander Ainslie said that he had arrived at the same 

 conclusions as Mr. Merlin with regard to the black-dot image. 

 He said that the use of the correct tube length tended to do away 

 with false images, and that the correct image was the evanescent 

 " black dot " with a wide cone of illumination and perfect tube- 

 length adjustment. Mr. N. E. Brown did not agree that diatom 

 valves were perforated. He thought that the black dot was the 

 result of the efiect of light on the thin convex or concave mem- 

 brane with which he believed the " dots" to be covered. He 

 did not believe it possible to get black- and white-dot images of 

 a hole. When Mr. Nelson showed him the secondaries of P. for- 

 mosum he had seen eight surrounding dots and more in the middle 

 of the primary, which was sunk in a shallow depression. The 

 thanks of the meeting were accorded to Mr. Merlin for his interest- 

 ing communication. 



Mr. Pledge was called upon to read his paper on " The Use of 

 Light Filters in Microscopy." Mr. Pledge said that the earliest 

 record of the use of a light filter in this connection was about 

 1704, when John Marshall made a microscope in which the eye 

 lens was made of smoked glass to render the colour fringes formed 

 by the simple objective less apparent. The matter appears to 

 have received no further attention till 1837, when Brewster 

 referred to the advantage of coloured screens for correcting 

 chromatic aberration. Adams (1787) records the use of oiled 

 paper or " glass lightly greyed " for modifying the intensity of 

 sunlight or the yellow colour of artificial illuminants. The 

 first application of light filtering applied to photomicrography was 

 by Kingsley in 1860. He used a solution of aesculine with an 

 oxyhydrogen light to absorb the ultra-violet, and so obtain 

 sharper results. The use of polarising apparatus and coloured 

 solutions and glasses was recommended by subsequent isolated 

 workers, but it was not till after the introduction of ortho- 



