371 



would find the depth in many of them was certainly greater than 

 the breadth. He had himself made a number of experiments 

 by the method of " interference colours," which he described some 

 time ago at the Club,* and had found in P. aiigidatuTn that the 

 depth exceeded the width. There were means of ascertaining 

 this which left no doubt upon the matter. Mr. Stokes had 

 referred to diffraction effects as being overwhelming. They were 

 of course always present, and their influence had been fully 

 recognised in his paper, but they were limited in the cases under 

 consideration to a very small area. As regarded the aperture of 

 the objective, it seemed rather like straying away from the subject 

 to bring this in, because it was always there under any circum- 

 stances, and the effect due to this was a very small amount. As 

 regarded comparing the hole in a diatom to a star disc as seen 

 through a telescope, he thought this was a somewhat far-fetched 

 comparison, because in the one case they were thr(jwing light 

 upon the object, and it depended upon the way in which the 

 Hght was thrown what the effect would be ; whereas in the case 

 of a star they had a self-luminous object, which was an unalter- 

 able condition of things. Pleurosigma angulatum was chosen 

 because its perforations were so very regular, and were just a 

 wave length apart ; but they could trace the effects very much 

 better with something more irregular and where they could 

 separate one thing from another. 



The thanks of the meeting were voted to Mr. Rheinberg for 

 his paper. 



A note by Mr. Merlin " On certain minute structure observed 

 in some forms of Triceratiuyn " was, in the absence of the author, 

 read by Mr. Karop : and the thanks of the Club were voted 

 to Mr Merlin for his communication and to Mr. Kaiop for 

 reading it. 



Mr. Karop read a paper on " Hanging-drop Cultivation," a 

 matter which had been referred to by the President in the course 

 of his annual address, and might not be well understood, at least 

 by some of those who were only beginners in microscopy. 



Mr- Bryce said he should like to congratulate Mr. Karop upon 

 his choice of a subject, and also upon the way in which he had 

 dealt with it. Papers of this kind were just what they wanted 



* " The Origin of certain Colour Phenomena typically shown by Actino- 

 cyclus Ralfsii" {ante, p. 13). 



