1064 PROCEEDINGS OF THE SOCIETY. 



difficulty, because, if mounted iu balsam, tbo finer details were obliterated, 

 and if dry only such portions of the valve could bo seen as adhered to 

 the cover-glass. By means of drawings on the blackboard, he explained 

 that the peculiarity consisted of a grating, each alternate hole of which 

 threw an image in a different focal plane, giving the effect when seen by 

 an achromatic condenser of a series of red beads with white spaces 

 between them. In the i)hotographs these beads came out square, and by 

 deeper focusing they appeared blue, and the valve itself was seen to be 

 somewhat hollow in section. Taken from that side, the valve showed 

 two distinct layers of structure, and from the other side ho found that 

 there was a very delicate membrane and a series of lines with a number 

 of fine rings stuck upon them ; this structure was very indistinct owing 

 to the interference of light in passing through the very fine membrane. 

 On focusing deeper a grating was seen ; but whether, in addition to 

 these three layers, there was any other structure, he was at present unable 

 to say. Referring to Dr. Carpenter's figures — which he reproduced on 

 the board — he inferred that the appearance there shown mu.st have been 

 obtained by using oblique light : he had seen an appearance something 

 like it, but could not say that this showed the real structure. He had 

 examined other species of this genus, and had found at least a double 

 structure in five other species — angulatuni, decorum, balticuvi, and two 

 species from Virginia. The photographs were all taken Avith central 

 light, and with as large an aperture as could be used without spoiling 

 the contrast. 



Mr. Ingpea inquired if Mr. Smith had used any mounting media 

 of a much higher refractive index, such as sulphide of arsenic, which 

 ■would produce contrasts in opposite directions, and in which, if he got 

 a happy fracture, ho might see what would cause him to modify his 

 opinions on some points. 



Mr. Smith said he had not tried them : he was not a good hand at 

 mounting, but he had tried them in Canada balsam without good results. 



Mr. Ingpen said that Canada balsam would be useless ; the diatoms 

 should be mounted in something of a higher refractive index than them- 

 selves to get as strong a contrast as possible. 



Mr. Karop said that one point which seemed to be overlooked was 

 that diatoms iu their fresh state were composed of silica in a colloid con- 

 dition, and the treatment of this by acids in the course of preparation 

 might account for a good deal of variation in the appearance of the 

 structure. 



Mr. Ingpen thought this was very probable, as it was impossible to 

 get a perfect resolution of headings or skeleton structure upon fresh 

 specimens, the only exception being perhaps in the case of Amphipleura, 

 which was so extremely delicate that, unless fresh specimens were 

 procured, the structure would be found greatly destroyed. In the case 

 of the stronger diatoms, it was necessary to submit them to acid or other 

 treatment to remove surface or internal substances, which otherwise 

 prevented what was supposed to be the complete skeleton from being 

 seen. 



Mr. J. Mayall, jun., said that one point had been touched upon 

 which he thought needed a little clearing up. It Lad become somewhat 

 fashionable for people to say that, in making their observations, they 

 ■were most careful to exclude all oblique light, and that their improved 

 ideas were due to the use of central light. Assuming that a condenser 



