285 



Mr. Stokes thought that if they examined a very simple 

 structure about which they had no doubt, the explanation was 

 easy to give ; but when they examined a diatom, the structure of 

 which they were not so sure about, the explanation became more 

 difficult. It was well known that in the case of a regular struc- 

 ture they got the black and white dots, but it would also be found 

 that the same effects w^ere produced in the same way on the edge 

 of a piece of silex, whether there were any perforations or not. 

 They were very thankful to Mr. Nelson for his remarks, because 

 his experience in these matters was very much greater than that 

 of any one he knew, 



Mr. Rheinberg said he should not be so rash as to make off-hand 

 any criticism on Mr. Nelson's paper, because it was well known 

 that Mr. Nelson's opinions were only given after so much study 

 that it must be assumed they were well grounded. It was said that 

 the perforations in diatoms acted as lenses, and that an image 

 was formed of anything through them. He thought, however,, 

 that in diatom structure they got tubes which were filled with a 

 substance of the same kind as the surrounding medium, and 

 he could not tell how these tubes could produce an image. It. 

 had not been suggested that the holes were in any way lecs- 

 shaped. In Actinocyclus Ralfsii the perforations were large^ 

 and the larger the perforations the larger they found the dots,. 

 If the perforations did give an image of the stop in the con- 

 denser it still seemed to him to want some sort of explanation^ 

 how this came about, and if so, how was it that they got three or 

 four images by focussing lower and lower. He hael a number o£' 

 slides mounted in about eight different media, and the result was. 

 that the black and white dots altered very considerably according- 

 to the media. The main point, however, was in what way these 

 perforations through the diatoms could produce images. 



Mr. Stokes said that a very simple experiment would show 

 how perforations could and did act as lenses. They had only to 

 make a small pin-hole in a piece of tin-foil, and it would be found 

 that this was cj^uite competent to produce an image ; and it was, 

 of course, well known that a pin-hole had been used for photo- 

 graphy, and very good results had been obtained. 



The Chairman was Cjuite sure that all who had heard Mr. 

 Nelson's paper would join in expressing their thanks to him for 

 it, and would regret that illness was the cause of his absence. 



JouRN. Q. M. C, Series II. — No. 50. 19 



