Sept. 19, 1 872 J 



NATURE 



423 



At a distance of one foot from the lamp the exposure required 

 is four or five minutes. The development is the most critical 

 part of the process. The pyrogallic solution ."should contain 

 plenty of acid (acetic or citric), and its action must not be pushed 

 too far, the mistake which a photogi-apher accustomed to nega- 

 tive work is most likely to make. At this stage the spectra 

 given by a candle flame are not very brilliant, on account of the 

 iodide of silver still covering the parts which are to be transparent. 

 Any trace of fog is especially to be avoided. I have experienced 

 advantages in many cases from a solution of iodine in iodide of 

 pot.assium applied to the film previously to fixing ; but its action 

 must be c.\refully watched, or too much silver will be converted. 

 The iodide of silver is then cleared away with hyposulphite of 

 soda or cyanide, followed by a careful washing under the tap. 



With regard to the gelatine copies, I have not much to add to 

 the account read before the Royal Society. The process is very 

 simple, and some of the results very perfect ; but I have not 

 hitherto succeeded in sufficiently mastering the details. Plates 

 apparently treated in precisely the same manner turned out very 

 differently. That difficulties should arise is not very extraordi- 

 nary, considering the novelty of the method ; but it is curious 

 that some of the very first batch prepared are among the best yet 

 produced. The value of the results is so great that I have no 

 intention of abandoning my attempts, and perseverance must at 

 last secure success. 



I will now say a few words about the performance and pros- 

 pects of the new copies. Their defining power on the fixed lines 

 in the solar spectrum is all that could be desired, being, so far 

 as I can see, in no way inferior to the originals. In the thivd 

 spectrum the 3,000 to the inch gratings show the line between 

 the D's, if the other optical arrangements are suitable. The 

 fourth line of the group, b, is distinguished with the utmost ease. 

 I am not sufficiently familiar with spectroscopic work to make an 

 exact comparison, but I presume that two prisms of 60' at least 

 would be required to effect as much. I am here speaking of 

 photographs on worked glass. With ordinary patent plate, 

 although very good results may be obtained, if tested by the 

 naked eye only, it is a great chance whether the magnifying 

 power of a telescope will not reveal the imperfect character of 

 the surface. 



With direct sunlight the light is abundantly sufficient, but it 

 is here in all probability that the weak point of gratings lies. It 

 should be distinctly understood that where light is deficient 

 gi'atings will not compete with prisms. There are cases, how- 

 ever, wliere the scale might be turned by the opacity of all highly 

 dispersive substances to the rays under examination. Even if 

 glass be retained as the substi-atum, it may be used in a very thin 

 layer, while prisms are essentially thick. The immense ad- 

 vantage of a diffraction spectrum for the investigation of dark 

 heat need not here be insisted on. Taking all things into con- 

 sideration, it is probable, I think, that photographed gratings 

 will supersede piisms for some purposes, though certainly not for 

 all. 



The specimens in the hands of Mr. Ladd are copies of two 

 gr.atings by Nobert, each of a square inch in surface, the one 

 containing 3,000 and the other 6,000 lines. The latter cost 

 about twenty pounds. 



SECTION G.— Geology 



Monday, Aug. 19. — On the Occurrence of Erect Bases or 

 Trunks of Psaronius in the Devonian Rocks of Neiu York, 

 U.S.. A., by Prof. James Hall. 



During the year 1870 some excavations were made in beds of 

 fine sandstone, referred at that time to the upper part of 

 Hamilton's group, but which probably belong to beds higher in 

 the series. In these beds several thousand trunks of tree-ferns 

 were found in an upright position, with their bases resting in and 

 upon a bed of clay, in which they appear to have originally 

 grown. In the clay, and in the sandstone above, to the height 

 of two or three feet, great quantities of vegetable substance oc- 

 curred. Principal Dawson refers these trunks to the genus 

 Psaronius, and he has determined two or more species from the 

 locality. 



These facts were held to indicate a point of comparatively dry 

 land upon the eastern margin of the Devonian Sea. Receding 

 from this ancient shore we find the sands and slates to become 

 finer, and the latter to change into calcareous mu-'s. For some 

 distance the shells occurring in these beds are all Lamellibranchs, 

 and it is only when we have travelled a considerable distance to 

 the westward that Braehiopods appear. The author then entered 



into some detail as to the mode of accumulation of these beds, 

 which are admirably exposed along a line of outcrop 300 miles in 

 length. 



Sur Ics Animau.x Fossiles du Mont Leberon, Vaucluse, by Prof. 

 A. Gaudry. 



The author remarked that the fossils found by him and others 

 in this place bore a striking similarity to those he had before 

 collected in Atiica. In comparing the 4,940 bones from the 

 latter place with the 1,200 from the former, he had been much 

 struck with the great variations exhibited by animals that seem 

 to descend from the same parent. The presence of numerous 

 and large herbivores proves the existence of a considerable ex- 

 tent of meadow-land during the Miocene period. 



Brief Notice of the Present State of our Knoialedge in connection 

 with the Brachiopoda, by Thomas Davidson, F. R.S. 



The object of this paper was "not to trouble you with 

 details, but to mention, in general terms, what has been 

 the advance effected in this portion of pala:ontology since 

 1853, the period at which I first published my general intro- 

 duction." Mr. Davidson first alluded to the general question 

 of classification, dissenting from the views of Prof. Morse, who 

 wished to remove the Brachiopoda from the MoUusca, and to 

 place them with the Annelides. The great importance of the 

 Brachiopoda to the palx'ontologist was then dwelt upon, and the 

 author remai'ked that ' ' many instances are on record where the 

 sight of a few specimens of Brachiopoda have enabled a paleon- 

 tologist to determine accurately the age of a rock in some distant 

 land to which he had no access." Mr. Davidson stated that the 

 number of so-called species of recent forms amounted to about 

 100, but that probably the number would have to be reduced 

 to about 60. " The number of so-termed species of Lingula 

 had been greatly exaggerated, and a certain number of the 

 others are known only by a single specimen." 



Tuesday, Aug. 20. — Mr. H. Vvoodward read his Sixth Report 

 on Fossil Crustacea. The report was illustrated by a large 

 number of diagrams, and announced the discovery of new 

 crustacean forms in Silurian, Coal Measure, and Permian rocks. 



Remarks onthe Genera Trinierella, Dinoboltis, and Monomerella, 

 by Thos. Davidson and Prof W. King. The authors proposed 

 to group these three genera into a new family to be called Tri- 

 merellidai. It[was shown to be structurally allied to the Lingulidje, 

 and it was inferred that the two families were genetically related. 

 This is a point of great interest, inasmuch as .the Lingulida.' are 

 the earlier forms, occurring in Cambrian rocks, whilst the Tri- 

 merellidx' first appear in the Silurian strata. The Cambrian 

 Lingulas have a horny shell, and so too have generally the ani- 

 mals associated them. In later formations the Braehiopods and 

 other animals have much more calcareous shells ; and from these 

 facts it was inferred that lime was less abundant in the Cambrian 

 sea than during later periods. As the result of long labour in 

 this field of research, the authors were led to adopt the doctrine 

 of evolution of species " effected mainly through the operation of 

 Divine laws, and not by purposeless or accidental modifications." 

 The paper concluded with detailed descriptions of the, structure 

 of the Trimerellids. 



The Rev. John Gunn's paper was then read On the Prospect 

 of Finding Productive Coal Measures in Norfolk and Suffolk, 

 with Suggestions as to the Places best adapted for an Experiihental 

 Boring. The author controverted the views expressed by Sir 

 R. Murchison as to there being no coal beneath these counties, 

 and on the following grounds: — The " Anglo-Belgian Basin," 

 in which the forest bed was deposited, is bounded by the chalk 

 on the west and south, and by older rocks on the east. It was 

 contended that this area has been characterised by vegetable 

 growths at several successive periods ; and that as regards the 

 coal-growths, these were accumulated in a basin bounded by the 

 carboniferous limestone, just as the forest-bed was accumulated 

 in a basin bounded by the chalk. Hunstanton was suggested as 

 the best place to bore ; because there the cretaceous rocks have 

 been denuded, and there too some of the oolites are absent. 

 Probably the bore would not exceed 1,000 feet in depth. 



An interesting paper was then read by the Secretary, forwarded 

 from Bonn by G. vom Rath, On a remarkable Block of Lava 

 ejected by Fesuzdus during the great Eruption of April 1872, 

 pnn'in:( the Formation of .Silicates by Sublimation. This was a 

 block of old lava which it is assumed had been floating in the 

 melted lava of the late eruption, and was subsequently ejected 

 by the volcano. It shows that in its interior there were fonned 

 crystals ofpyro.xene, mica, sodalite, specular iron, and magnetite; 



