58 PROCEEDINGS OF SOCIETIES. [^o?m?l JiSiTi? ^ 
of hexagonal structure, and that this appearance was due to the direc- 
tion of the shadow in the one case producing the effect of elevations, 
in the other of depressions. From this appearance his brother had 
argued very strongly, in regard to the markings of some diatoms, that 
they were caused by hemispherical nodules. 
He (Mr. J. Beck) had examined some specimens of silex artificially 
deposited, and amongst the more delicate fragments were pieces 
exactly resembling Pleurosigma in structure; and on examining the 
coarser fragments, which were similar to the more delicate ones in 
structure, he found them to consist of hemispherical nodules deposited 
at regular intervals on siliceous plates, showing clearly that an 
appearance similar to that seen in the Pleurosigma could be produced 
artificially, and affording a strong argument that the markings were 
due to hemispherical nodules lying on a siliceous plate. At the same 
time he did not consider the difficulty solved, for the markings on 
some of the larger Diatomacese, as Isthmia, Triceratium, Pinnularia, 
&c., could not be accounted for in this way. The difficulties of 
ascertaining real structure from the appearance presented might be 
seen by taking a piece of zinc pierced with round holes and allowing 
the sun-light to pass through it upon a sheet of paper ; if held at 
a certain distance there would be an image of round holes ; but if the 
paper were removed or brought closer to the plate, the appearance of 
black hexagonal dots or of light hexagonal interspaces would be pro- 
duced. He did not wish for one moment to undervalue the discovery 
of the President ; but he thought more knowledge of the structure of 
an object could be obtained by the use of a variety of modes of illu- 
mination than by the observer restricting himself to the use of one only. 
Dr. Eulen stein thought the structures observed in the larger 
diatoms quite different from some of those of which the President 
had spoken. As regards, however, the nature of the markings on 
the Pleurosigma angulatum, he had, after examination with Powell's 
J^th and Hartnack's immersion lens, observed that the character of 
the dots was hemispherical, a conclusion which Hartnack had dis- 
puted. But he was not disposed to believe that the dots were com- 
plete spheres of silex. 
Mr. Hogg said he had, in conjunction with Mr. Mayall, paid much 
attention to the subject in question, and he had come to the conclusion 
that the markings in question were not merely dots. Hartnack was 
now quite willing to admit that he had been in error with regard to 
the diatom markings, and that they were really raised spherical dots. 
Mr. Lobb said that for years he had considered the markings of 
the Pleurosigma spherical, and indeed had never looked upon them as 
anything else ; and he was glad to find that his belief could be con- 
firmed by the use of a low power, and at so little expense. 
A vote of thanks to the President was then unanimously passed. 
Mr. Hogg then read a paper "On the Results of Spectrum 
Analysis." 
Mr. Hogg also informed the Fellows that Dr. Herapath, a short 
time before his death, had been engaged in making investigations into 
the spectra of the chlorophyll of vegetable substances, of which he 
had examined about 250. He (Mr. Hogg) had hoped that these 
