u 



the colours shown by the diatom Actinocyclus Ralfsii described 

 by Mr. E. M. Nelson in his paper read before the Club on 

 June 15th, 1900. 



Mr. E. M. Nelson said he was extremely sorry that by 

 dangling this colour before Mr. E-heinberg he had caused him 

 such uneasiness ; but he thought the members of the Club would 

 all be glad that he did so, since it had been the means of bringing 

 them a very excellent paper. His reason for mentioning the 

 subject was that he had found this diatom to behave differently 

 from other diatoms ; but Mr. Rheinberg now told them that by 

 putting a diaphragm over the objective other diatoms were also 

 found to be coloured. The paper was obviously one which could 

 not be fairly discussed without reading it ; but he thought that 

 when the thickness of the diatom was considered, together "\vith 

 the fact that diatom silex was so nearly the same as to its 

 refractive index as Canada balsam, it was not at all easv to 

 account for the effects seen. It seemed to him that the colours 

 of diatoms differed from those of Newton's rings, and had a 

 different origin. He should certainly try the experiment sug- 

 gested, and he was glad Mr. Rheinberg had demonstrated that 

 this colour existed in the case of other diatoms. 



The Chairman said he was sure their thanks were due to 

 Mr. Rheinberg for this paper, which was certainly one, as Mr. 

 Nelson had said, which could only be discussed after reading 

 it ; indeed, he was not quite sure that he should feel able to 

 discuss the questions raised even after reading it. 



The thanks of the meeting were, on the motion of the Chairman, 

 unanimously voted to Mr. Rheinberg for this communication. 



Mr. Rheinberg said lie should be very pleased to show any one 

 the colours in other diatoms beside Actinocyclus, and, as already 

 mentioned, he thought that the colour could not be due to diffrac- 

 tion. As regarded Newton's rings, he did not intend it to be 

 understood that these phenomena were the same as those of 

 Newton's rings, but rather that they were due to similar causes, 

 as in this case the extinctions always recurred in the same order. 



The Secretary reminded the members that at their next ordinary 

 meeting it would be necessary to nominate members of the 

 Committee for election at the annual meeting in February, and it 

 would also be necessary to appoint some member to act as auditor 

 on behalf of the Club. 



