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he felt himself at a great disadvantage through not being himself a mounter. 

 He quite agreed as to the utter uselessness of sections of diatoms as contributing 

 at all to a knowledge of their structure. The proper thing was to take some 

 ordinary broken portions, and to make what they could out of them. The 

 best lens for the purpose of these investigations was one in which the correc- 

 tion was such as to show a slight rose colour. This would show the thicker 

 membranes coloured red or rose colour. He had been looking at a specimen 

 that day on the surface of which were a number of exceedingly minute 

 spines ; these all looked rose colour. He quite agreed with the remarks as to 

 the exceeding beauty of specimens of Araehnoidiscus. It was, he thought, 

 one of the most lovely objects that could be found ; the beautiful dove-tail- 

 ing; in of the various parts of the pattern was a structure the perfection of 

 which was marvellous. 



Mr. Deby said he did not know that there was to be a discussion upon 

 this subject, but he had prepared a short note " On the Structure of Diatom 

 Valves," which he proceeded to read. 



Prof. Stewart said that perhaps the remark which Mr. Deby had just 

 made would apply to himself, and it might be that others had led him into 

 error upon this subject. It was a good many years since Mr. Stephenson 

 made some preparations of diatoms, in phosphorus dissolved in bi-snlphide 

 of carbon, and the appearance which they presented was something like that 

 which he drew upon the black board. If it really had that form he should 

 have thought it would have produced the same effect as a bi-coavex lens, and 

 that it should alter materially in its focus according to the medium in which 

 it was placed. But what they really found was that, whether they examined 

 it in a medium of high or low refractive index, the effect was that of a 

 number of little lenses, and so far as he Avas aware only a series of holes 

 could act in this way. Certainly they did act as if they were a series of 

 convex lenses, each one producing an image. He had never tried the 

 experiment with hydro-fluoric acid, but from what he had seen and heard 

 he was under the impression that they were actual holes. 



Mr. Deby said that these spaces were never filled by air or by dirt, and if 

 they were holes he should expect them to be so frequently ; but as a matter 

 of fact there never was an}-. There was a flat top not curved on either 

 side or thickened at the corners, and there was a hole in the bottom mem- 

 brane, as he drew it on the board III. Could Prof. Stewart say what would 

 be the diffraction effect of such an arrangement? 



Prof. Stewart imagined that an image would be formed by it. 



Mr. E. M. Nelson thought this was hardly the point at issue. The question 

 was not one of diffraction effects, but of making images. With Pleurosigma 

 images could be made, as he had himself shown in that room. 



Mr. Deby said it seemed to be a matter of facts, and therefore he would 

 bring down some of his slides, and would show them at the next meeting. 



Prof. Stewart said perhaps Mr. Deby w r ould do them the favour to bring 

 also some of those which showed the shape he had drawn upon theboard. 



Mr. Deby said he should be very pleased to do so. 



Prof. Stewart said as a matter of probability he should not have thought 



