1871.] 



Light. 275 



something very like the ! markings of the Podura test scale, Lepidocyrtus 

 curvicollis. The appearances in the photographs are much more deceptive 

 than those seen in the glass rods themselves, as the observer is aware of what 

 he is looking at. The effect is most conveniently produced by mounting a 

 number of small glass rods placed in contact in two circular frames, which are 

 caused to revolve over each other by the contrivance used in chromatrope 

 slides. The deceptive appearances are very well shown by using the slide as 

 a magic-lantern object. 



The fossil sponge spicules obtained by Mr. W. Vicary, of Exeter, from the green- 

 sand of Blackdown and Haldon, and described by Mr. E. Parfittin the "Trans- 

 actions of the Devonshire Association for the Advancement of Science," have 

 been further examined by Mr. H.J. Carter, F.R.S., and form the subject of apaper 

 in the " Annals of Natural History," vol. vii., p. 112. The spicules are embedded 

 in quartz sand, and are, as Mr. Carter believes, the remains of dead and disin- 

 tegrated sponges, consisting chiefly of the larger spicules, more or less in a 

 fragmental condition, and altered in shape by trituration ; the minute forms of 

 spicules are generally absent. The surface of the spicules is considerably 

 eroded, and presents the peculiar form of chalcedony. The triturating and 

 solvent influences attending petrifaction have probably obliterated any spinous 

 or tubercular processes which might have been expected to have been found on 

 spicules of so large a size, and which exist on recent sponge spicules with 

 which they are identical. Besides sponge spicules, the deposit contains a few 

 minute bivalve shells and Foraminiferce, but no remains which could be 

 identified with the calcareous spicules of Echinodermata, Alcyonidce, Gorgonidce, 

 or AscidicE. Owing to the tolerably perfect condition of many of the spicules, 

 Mr. Carter has succeeded in identifying a large number of them ; but owing to 

 the almost entire absence of the smaller spicules, it has only been possible to 

 make the comparisons with the larger spicules of recent sponges. The four 

 plates which illustrate the paper contain seventy-six figures of spicules from 

 the localities named. 



Mr. H. J. Slack has noticed the unusual form of microscopic crystals pro- 

 duced by dissolving various salts in a colloid solution of silica obtained by 

 dialysis. Sulphate of copper afforded the following results i — First, what may 

 be termed a " pavement pattern," in which the silica cracks divide the film into 

 a great number of compartments, of three, four, five, or more sides, which may 

 be roughly likened to the appearance of vegetable ivory, or a section of pinna- 

 shell. In each of these divisions crystallisation takes place, usually radiating 

 from the centre, and sufficiently diversified in thickness to produce polariscope 

 patterns. The crystals thus produced have a tendency to exhibit Maltese 

 crosses, of colour changing with the positions of the polarising and analysing 

 apparatus. In these " pavement patterns," the force exerted by the silica film 

 in contracting and cracking causes the crystalline force to operate in limited 

 compartments without change in the linear direction common to radiating 

 groups. In-other cases, the contractile force acts tangentially, or in curves, 

 while the crystalline force operates radially, and then exquisite spiral and 

 turbinate forms, similar to those described by Mr. R. Thomas (Quart. Journ. 

 Micr. Sci., 1866, p. 177). Hippuric acid, salicine, and tartaric acid, all yielded 

 patterns differing from the ordinary forms of crystallisation from aqueous solu- 

 tions. Mr. Slack remarks, in conclusion, that it is impossible to view a number of 

 slides prepared in this way without being struck with the resemblance of many 

 of the patterns to those of sections of various structures in organised beings, 

 and we are led in the direction long since indicated by Mr. Rainey, to extend 

 our notions of the functions performed by the chemical and physical forces at 

 work according to their own methods in living things. The action of the silica 

 appears to be chiefly physical and mechanical; but the appearance of the 

 crystals leads to the belief that some portion of silica is present in them, and is 

 not eliminated as the, process of crystallisation goes on. (Monthly Micro. 

 Journ., vol. v., p. 50, and pi. 7.7 and 78).' 



A new microscrope lamp, designed by Mr. Fiddian, of Birmingham, has 

 recently been brought forward by Mr. Browning. One of its merits is great 



