March 14, 1878] 



NATURE 



379 



of short length. A description of the remarkable negative 

 or reversing action effected by the red rays on the sen- 

 sitised plate, first observed by H, Draper, is found in 

 Chapter XXXIV. A partial explanation of this very in- 

 teresting fact is given by the results of experiments lately 

 made by Capt. Abney {Phil. Mag., January, 1878), which 

 show that the image can be rendered undevelopable by 

 the oxidation of the altered silver compound forming it. 

 Chastaing has also recently announced that he finds 

 rapidity of oxidation promoted by the red rays. It is 

 thus easy to see that the sensitive salt of silver which had 

 been altered in chemical composition by a slight exposure 

 to white light, would become oxidised where the red rays 

 fell upon it, and that, in consequence, where the dark 

 Fraunhofer's lines in the ultra red spectrum fell, the plate 

 would remain unaffected and the presence of these in- 

 visible bands would become apparent. 



Another subject of great interest, that" of the production 

 of coloured photographic images, is being attacked ex- 

 perimentally by Capt. Abney. The results of the experi- 

 ments in this direction by Becquerel and Nicpce de St. 

 Victor are well known, and many of the visitors to the 

 Loan Exhibition will remember the coloured photograph 

 of dolls dressed in coloured clothes shown by the latter 

 chemist. Abney believes that these tints are rather to 

 be ascribed to different stages of oxidation of the film, 

 than, as has hitherto been supposed, to the colours of 

 thin plates. Then, again, on the subject of the recent 

 discoveries by Vogel, Waterhouse, and others, as to the 

 production of a film sensitive to the red rays by the 

 addition of a red dye to the collodion, Capt. Abney has 

 something original to say. He has found that the addition 

 of certain resins, albumin, and other organic bodies, when 

 combined with silver, tends to lower the limit of the 

 impressible spectrum and the place of maximum sensi- 

 bility ; so much so, indeed, that it is possible to obtain 

 an unreversed impression of the thermal spectrum. A 

 beam of light was allowed to pass through ruby glass, 

 and the spectrum was then thrown on a resinised plate 

 in the ordinary manner, and a visible impression of rays 

 in the red was obtained far beyond the limit of the visible 

 spectrum, as is seen by a figure in the volume. 



Enough has been said to show the value of Capt. 

 Abney's treatise both from the scientific and artistic 

 points of view. If we are to speak on the part of amateur 

 photographers we would express a hope that the subject 

 of the explanation of defects in negatives and their cure 

 may be more fully treated of in the next edition. It is 

 perhaps difficult for an accomplished photographer like 

 the author to appreciate the difficulties of a beginner in 

 the art, but the mere mention of some of the defects met 

 with in negatives does not always, as the author states, 

 suggest the cure to minds unfamiliar with the niceties of 

 manipulation and procedure which to the expert come as 

 a matter of course. We congratulate Capt. Abney on the 

 appearance of this most useful volume. H. E. R. 



OUR BOOK SHELF 



ArchcEolo^ical Researches at Carnac, in Britanny. By 

 James Miln. (Edin .urgh : David Douglas.) 



This beautiful book reflects great credit on its author. It 

 would be difficult in the recent literature of archaeology 

 to point out a more salient example of the great gain 



which is sure to accrue to that branch of science from the 

 introduction of the true scientific spirit, and attention to 

 details. Carnac, in most people's minds, is associated with 

 Druidical circles, and it was to see the wonderful align- 

 ments there that Mr. Miln visited the place. But while 

 in the region the author was particularly struck with the 

 remains belonging to a very different time, which were 

 pointed out to Mr. Miln by a French archaeologist. They 

 are termed the mounds of the Bossenno. With charac- 

 teristic energy Mr. Miln, who was determined to explore, 

 endeavoured to buy in order that he might explore the 

 better. In this, however, he was foiled, beset by too 

 many difficulties. The permission to explore which he 

 subsequently obtained does not appear to have been a 

 very complete one, and after this big book full of matter 

 our author states that much still remains to be done. 



The results of the excavations so carefully carried out 

 by Mr. Miln show that we have here the remains of a 

 Gallo- Roman settlement, and he has reconstructed for us 

 out of its ashes the condition of the people in former 

 times. He has been enabled to give us precise informa- 

 tion as to their food and the degree of luxury in which 

 they indulged. Their worship, their ceremonies, and 

 modes of manufacture, and the exact times between 

 which the colony was in a flourishing condition are also 

 fully discussed. He traces the local worship of Venus 

 Genetrix, at the Mont St. Michael, in a most interesting 

 manner. One of the oldest constructions which remains 

 in Britanny is the chapel of St. Agatha. On the vault of 

 the apse a few years ago was discovered one of the most 

 curious frescoes which the Romans have left in Britanny. 

 It represents Venus rising from a blue sea, surrounded 

 by fishes and dolphins. This church, now dedicated to 

 St. Vener, is styled " Ecclesia Sancti Veneris " in a 

 twelfth century charter. 



The beautiful illustrations comprise not only almost 

 everything which was found, but large coloured plates of 

 the chief coloured designs rescued here and there. 



All antiquaries will do well to lay to heart the remarks 

 on ancient pottery made by Mr. Miln ct propos of his 

 finds in the excavation which he designates A. He shows 

 abundantly how much caution is requisite in such in- 

 quiries and how a careful sifting of facts brings order into 

 what at first sight appears a hopeless jumble of objects. 

 It is curious that some of the pottery he found there is 

 similar to some in the Guildhall Museum, which was 

 found at a depth of forty-two feet, when the ground was 

 excavated for the foundations of the Royal Exchange. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers op, refected manuscripts. 

 No notice is taken of anonymous comtnunications. 



[ The Editor urgently requests correspondents to keep their letters ar 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts."] 



The Telephone 



In his interesting paper (Nature, vol. xvii. p. 283) Mr, 

 F. J. M. Page communicated as the result of his experiment 

 to obtain indication of currents from a telephone by means of a 

 mercury capillary tube, that the motion of the mercury was 

 "always towards the end of the capillary." In the repetition of 

 this experiment before the Physical Society on Saturday, 

 February 16, Mr. Page found, however, that the mercury moved 

 persistently in the opposite direction. 



In the December (1876) number of the Phil. Mag. I showed 

 that the motion of mercury in contact with dilute acid through 

 which a current passes, is due to rapid circulation of the mercury 

 set up by deoxidation of one part of its surface whilst another 

 part is being oxidised ; and that a very slight difference in the 

 degree of oxidation is sufficient to produce an appreciable electro- 

 motive force. 



