PROCEEDINGS OF SOCIETIES. 
281 
believed they were usually depressions not elevations, but thought 
care must be taken to distinguish those belonging to the outer silicious 
lorica from others appertaining to the lining, or veil, which veil pro- 
bably differed chemically and structurally from the rest of the valve. 
The method adopted was one recommended by an American 
observer, and published in the ‘ Annals of Natural History ’ some years 
since. It depended on the relative indices of refraction between the 
diatom and the medium in which it is mounted. The silicious 
deposits in both animals and plants are of less index of refraction 
than Canada balsam. When mounted in this medium, they, if convex, 
will really act as a concave lens would in air, and vice versa. To 
prove this examine a silicious and a calcareous spicule of a sponge in 
water and then in balsam. In water they will both appear bright when 
beyond the focus of distinct vision, and dark when within it ; in 
balsam, however, the calcareous spicule will appear as before, but the 
silicious one will be dark when beyond the focus. In this experi- 
ment the compound eye of a beetle may be conveniently substituted 
for the calcareous spicule. 
If now one of the larger diatoms be examined it will be found to 
behave in the same manner as the beetle’s eye or calcareous spicule, 
thereby showing that if the diatom is of the same refractive power 
as the silicious spicule (which can be proved by other means) its 
markings must be depressions. The veil, however, is probably of 
greater index of refraction than the rest of the valve, and if this be 
the case the same method shows the markings in Heliopelta to be 
elevations. 
Mr. Slack said he thought the markings were spheroidal, from an 
examination of a great many fractured valves, and from seeing some 
of the larger spheroids as lenses. He drew some analogy between the 
real diatom and Max-Schultze’s artificial diatoms ; and thought that 
the principle of chemical precipitation was similar in both. He was 
very much inclined to think from the examination of a great number 
of objects of the class in question that all were constructed on the 
same type. The vegetable matter of the diatom acts chemically upon 
the silex in the water, which is the condition of colloid silica, and the 
deposition always takes place in the spheroidal form. 
Mr. Brooke would like to know how it happened, if the structure 
of these objects were really hollows and not bosses, that the line of 
fracture runs between the dots, and not through them. He had frac- 
tured a great many diatoms under the microscope, and had never seen 
a single instance in which the line of fracture did not run equidistant 
between two consecutive dots, which would hardly be the case if they 
were depressions and not elevations. 
A vote of thanks was then passed to Mr. Slack. 
Mr. B. T. Lowne read a paper “ On the Nervous System of Insects 
and Crustaceans compared with that of Annuloida and Vertebrata.” 
Mr. Ingpen called the attention of the Fellows to a marvellous 
photograph of a solar spectrum made by Mr, Browning from a litho- 
graph of Fraunhofer, which he thought was likely to prove of great 
value in spectrum analysis. 
