173 



already passed a formal vote of thanks to Mr. Mot tram, but he 

 had not the slightest doubt that every member of the Club would 

 like to join in this. 



The thanks of the meeting were then unanimously voted to 

 Mr. Mottram for his donation, and also to the donors of the 

 various additions to the Library. 



Mr. W B. Stokes read a paper on " Images of Diatom 

 Structure," the subject being illustrated by diagrams drawn upon 

 the blackboard. 



Mr. J. Rheinberg followed with a paper on the same subject, 

 entitled " The Black and White Dot Phenomenon," in which the 

 effects described by Mr. Stokes were explained to be the results 

 of entirely different causes. 



Mr. Stokes, replying to a portion of Mr. Kheinberg's remarks, 

 pointed out that it was the late Mr. Haughton Gill who filled up 

 the perforations in the diatoms with solid matter, and it was then 

 found that the " black dot " of these diatoms did not occupy the 

 same plane as before. What he was trying to show was that 

 these appearances followed some general rule, and that if they 

 changed the zones of the objective so that the refracted light and 

 the reflected light changed places, the position of the dots was 

 altered. If three zones were used, then they had three images. 

 He thought that the trellis-work system referred to by Mr. 

 Kheinberg would produce an almost indefinite effect, not following 

 any rule. 



Mr. Kheinberg said he perfectly agreed with Mr. Stokes that 

 these things followed a certain rule, but he did not think it had 

 anything to do with the objective, because when using an apo- 

 chromatic he could get one and even two images above the focal 

 plane, and nearly always two below. His reference to trellis- 

 work was merely by way of crude analogy. There was, of course, 

 no true trellis-work in the structure, t'ut there was such an 

 amount of overlapping that it was not possible to get a distinct 

 image of what was underneath. They were not only in this case 

 dealing with the rays mentioned, but they had a number of others 

 still more oblique crossing each other in all directions and causing 

 irregular effects. 



On the motion of the President, a hearty vote of thanks was 

 given to Mr. Stokes and Mr. Kheinberg for their respective 

 communications. 



JouRN., Q. M. a, Series II.— No. 49. 12 



