52 PROCEEDINGS OF SOCIETIES. 
use. He knew there were some, and amongst them was Professor 
Abbe, who went so far as to say that the results obtained by the use of 
high powers on diatoms were entirely fallacious, and he had prepared 
a number of diffraction slides to prove it. He had himself seen these 
experiments at his own house, and he hoped that there would be an 
opportunity for them to be shown to the Fellows of the Society. But 
although it might be possible to produce very remarkable appear- 
ances by means of a number of gratings ruled in particular ways, 
this would not prove that they could not also get effects of similar 
aspect due to real structure. Whether these appearances could be 
relied on or not, was a question which a series of diatoms such as 
those before them gave very great help in solving, for they saw there 
a large number of specimens of the same species differing only in 
what might previously have been expected from their difference in 
size, and when they got this effect they might be tolerably snre that 
there was some sort of reality about it. He received some time ago 
a slide from Mr. Baker on which he could show in the same field of 
view a valve of Rhomboides with the rows of beads running as Mr. 
Dallinger showed them, and another which showed them running in 
a serpentine form. It was quite certain in this case the latter were 
spurious, but there was no reasonable doubt that the others were 
real. He did not know whether a drawing of Dr. Pigott’s had been 
shown there which had some bearing on the subject. Dr. Pigott was 
making some observations upon these spurious images by directing 
his telescope upon the bulb of a small mercurial thermometer, and he 
found that when the sun shone upon it he could see nothing but 
diffraction rings, but when a cloud came over the sun he could see 
a perfect reflexion of his own house. He had himself noticed a 
similar effect when trying to see with his telescope the two little com- 
panions of the Pole star; when diffraction rings were seen plainly, 
these small stars were invisible; but on a fortunate evening, when 
there was much light in the sky, they were seen quite distinctly. 
Dr. G. C. Wallich said he had observed that the size of the 
diatoms depended upon the size of the first frustule which grew from 
the sporangium in any given pool. He had repeatedly found this to 
be the case in India, where they grow very rapidly; the result being 
that all which came from the same pool were the same size. They 
would find in the collection in the Society’s cabinet * some gigantic 
specimens of LN. crassinervis in which the markings were very coarse. 
He was excessively glad to hear what Professor Abbe has said on the 
subject, because it was exactly his own opinion, for the expression of 
which he had got rather severely handled some years ago. 
Mr. Ingpen suggested that, after all, the question of genus and 
species depended upon difference in shape, and not upon the absolute 
number or fineness of the lines. He had not paid much attention to 
the subject, and therefore did not speak with authority ; but in the 
specimen of NV. crassinexvis there were decided nodules at the end of 
the median line which the other closely allied forms did not possess. 
He could hardly say that the specimens drawn by Dr. Dallinger were 
* Presented to the Society by Dr. Wallich. 
