18 THE PRESIDENT'S ADDRESS. 



and Mr. J. W. Gilford, who has also done excellent work in this 

 direction,* sent me subsequently a very fine fluid screen 

 of malachite green and picric acid. These were the screens 

 you heard about last year. Since then Mr. Lovibond has sent 

 me another sample of bluish green pot glass, which cuts the 

 red out more thoroughly than the peacock blue glass. Mr. 

 Gifford also has given me two more fluid screens, one a green 

 and the other a violet. f The violet screen is too dark for 

 visual purposes, and as yet I have not experimented with it 

 photographically, but the green screen passes a large quantity 

 of light. The results obtained with it are certainly up to, and 

 probably superior to those with monochromatic light from my 

 apparatus of the most approved pattern. There is one very 

 important point with regard to the use of screens for visual 

 purposes, viz., that if they are too dark they will obliterate 

 fine detail, and it will be found better to pass a wider band, 

 even should it contain some objectionable rays (i.e., not so 

 monochromatic), than cut down the intensity of the light too 

 much. In fact, there is a happy medium between the 

 monochromatism of the screen and the light intensity; for 

 example, Mr. Gifford's new green screen, which passes all 

 the green and a great deal of blue, is a much better screen 

 than that of Prof. Zettnow, which is more monochromatic. 



Before entering on the subject of the evening, a few words 

 must be said regarding a pamphlet published last year by Mr. 

 Allan Dick, entitled, "Additional Notes on the Polarizing 

 Microscope."! Most of us are acquainted with his former 

 excellent pamphlet, and with the improvements he has made 

 in petrological microscopes. This new work, however, deals 

 largely with the manipulation and management of an ordinary 

 microscope, quite apart from petrological work. It is to this 

 portion of the book that we must, therefore, confine our atten- 

 tion. 



In the book is described a new method for measuring the 

 aperture of a lens, which depends on the number of rings that 

 can be counted in a biaxial crystal when viewed under a wide- 

 angled axial cone of polarized light. The angles of the ring 



* "Journal R. M. S.," 1894, p. 164, and PI. V., Pig. 4. 



t An account of these will shortly appear in the " R. M. S. Journal/' 



X Published by Messrs. Swift & Son, 81, Tottenham Court Road, W. 



