PROCEEDINGS OV THE SOCIETY. 749 



a very interesting extract from the American Quarterly Microscopical 

 Journal, Vol. I., No. 1, published at New York in October 1878, in 

 which Professor Wm. Lighten had proposed the use of a very minute 

 aperture placed in the Ramsden disk for the purpose of cutting down 

 the diameter of the transmitted beam and so obtaining oblique or other- 

 wise modified illumination, but observed that the Professor did not seem 

 to have noticed the advantages resulting from the use of a central stop 

 in that position. 



Mr. Rheinberg said that, in considering Mr. Gordon's paper, of 

 which he had had the advantage of seeing an advance proof, he 

 thought the main point we had to keep before us was that the use of a 

 stop in the Ramsden circle of the Microscope was from an optical point 

 of view precisely equivalent to the use of a stop in the upper focal plane 

 of the objective. 



That any opaque spot which put out of use the central portion of 

 the objective, and cut off the whole or a portion of the direct light 

 beam from the condenser, deteriorated and falsified the image, had been 

 so often demonstrated and proved theoretically and practically during 

 the last thirty years that it seemed to him to be waste of time to go over 

 the ground again. To contend differently was to contend against the 

 elementary principle, which was common to all theories of the Microscope, 

 that image formation in an optical instrument was brought about by the 

 reunion of a multitude of light rays, from the object points, bearing 

 definite phase relationship to one another. One could not with iim- 

 punity cut out the central portion of these rays, without gravely dis- 

 turbing the result in the neighbourhood of the true image points, for 

 one is liable to get triplication and duplication of lines and intercostal 

 markings, besides the intrusion of other detrimental features. 



He himself had warned against some of the damages in his paper on 

 colour illumination, before this Society about ten years ago, and that 

 evening he had on the table a slide of familiar diatoms, and also a 

 grating (Grayson's band plate) shown under the Microscope with an 

 objective and Mr. Gordon's top stop apparatus fitted with opaque stops, 

 from which they could see for themselves what false and incorrect 

 images were produced. But better examples of these false effects could 

 not be found than those which Mr. Gordon had himself shown them on 

 the screen that evening in the case of P.formosum and the slide of 

 bacilli. 



Curiously enough P. angulatum formed, under circumstances, some- 

 what of an exception. The reason for this could be readily enough 

 explained, though he would not detain the Meeting by entering into 

 them that evening. 



Now, whilst it seemed to him that the application of the top stop 

 apparatus as advocated by Mr. Gordon was entirely misplaced, he for one 

 was grateful to him for having devised the ingenious little piece of 

 apparatus itself. It lent itself admirably to various kinds of optical ex- 

 periments, but chiefly, he feared, to those which disproved his own ideas 

 — for all the Abbe experiments in connection with the diffraction theory 

 could be repeated with it. It would also, he hoped, turn out to be a 

 convenient apparatus for colour illumination by the diffraction method. 



Dec. 19th, 1906 3 d 



