July 8, 1897 J 



NATURE 



223 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of rejected 

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications.'\ 



Colour Photography. 



I HAVE had a good many inquiries addressed to me about the 

 Dansac-Chassagne process of colour photography, described in 

 Nature, February 4, 1897, and as I was to a large extent re- 

 sponsible for having drawn public attention in this country to 

 the process, I am naturally anxious that those who feel an 

 interest in the subject should have, at all events, such information 

 about it as I can supply without a breach of the confidence with 

 which I have been favoured. 



First let me say that the information I can publish does not 

 include any details as to the materials employed, or the method of 

 their manufacture. The process is to be worked as a secret one, 

 and the statements made to me as to the method of preparing 

 the liquids used were confidential. This of course greatly affects 

 the scientific interest of the question, but commercial considera- 

 tions in this, as in many like cases, overpower scientific interest. 



It is only quite lately that I have been able to make any 

 fuller examination of the process, and such rough tests as I have 

 made are very incomplete. They are, however, complete enough 

 for me to say that, while they do not justify the claims origin- 

 ally put forward as to the completely automatic nature of the 

 process, I yet cannot account for the results produced without 

 admitting that the selective action claimed does to some extent 

 exist ; that the colouring matters applied to the photographic 

 print have a certain tendency to attach themselves to those 

 portions of the print which would be of a similar colour had the 

 image been reproduced in natural colours, and also that the 

 colouring matters, when applied in succession, do so combine 

 or react on each other as to reproduce, approximately, the tones 

 or tints of the original. 



There are some other inaccuracies in the first description 

 given to me, which I should like to correct. It does not appear 

 to me that a special negative, or a special print, is any advant- 

 age. It certainly is not necessary. The process can be applied 

 to any ordinary print on albumenised paper, or to any positive 

 on an ordinary gelatine plate. It does not appear to work 

 successfully with some, at all events, of the more modern 

 printing-out papers. Nor is it a fact that during the process of 

 treatment the positive has to be exposed to bright light. 



This it will be said is different from the process as it was first 

 described. Precisely. It is because of that difference, and be- 

 cause I was responsible for the publication of the first description, 

 that I now ask you to allow me to inform your readers as to the 

 real value of the process so far as I can estimate it at present. 



The above conclusions are based on my own attempts, and 

 some rather more successful) by my friend Mr. Herbert Jack- 

 son. Judging from the work of the skilled operators, whom I 

 have watched in M. Chassagne's studio, I can only say that in 

 their hands the process is certainly not entirely automatic. The 

 operator requires to know generally what the colours should be, 

 and the results largely depend on his judgment and skill in 

 applying the colour in the right places. 



But the practical outcome is that anybody, after a little in- 

 struction, can produce, with very great rapidity, coloured pic- 

 tures which, as evidenced by the specimens shown in public, are 

 of considerable merit. The result is obtained by first applying 

 the liquids over the whole picture, and then working up the 

 different parts of the picture by applying them locally. About 

 the truth of this there can be no manner of doubt. Hundreds 

 of persons in Paris have seen it done, and have admired the 

 results. The difficulty is to satisfy oneself as to how far the 

 process is purely mechanical, and how far it is a matter of skill. 

 A certain amount of skill is required, but, admitting this, it 

 appears to me that taking it at its lowest value, the process does 

 provide a means of colouring photographs — and with approxi- 

 mate correctness — that has not previously been available. 



Whatever may be the practical or commercial value of the 

 process, it will prove of very great theoretical interest, if as an 

 outcome it should be conclusively proved that any monochrome 

 photograph has even the smallest power of colour selection, 

 depending on the tints of the original ; as this once established, 

 some of our current photographic notions would be revolu- 

 tionised. Henry Trueman Wood. 



NO. 1445. VOL. 56] 



Telegraphy without Wires, and Thunder-storms. 



I have recently made experiments on telegraphy without 

 wires, and during the last few days, which have been very hot, I 

 have experienced certain phenomena which somewhat interfere 

 with the reception of signals. The receiver used by me is con- 

 structed thus : a piece of goose-quill one inch long is stopped 

 at each end with cork, two ordinary pins pierce each cork, 

 their points being about ^-inch apart ; the intermediate 

 space is filled with finely-powdered nickel ; (experiments 

 on the relative sensitiveness of receivers made with quill 

 and glass show that those made with quill are more 

 sensitive than those made with glass.) The receiver is at- 

 tached to an acoustic resonator carrying an electrically driven 

 tuning-fork {500 double vibrations per second). The receiver 

 is placed in contact with the foot of the fork, where it is attached 

 to the resonator. The receiver forms part of a circuit including 

 a dead-beat galvanometer, a single dry cell, and a resistance of 

 1500 ohms. One terminal of the receiver is earthed, and the 

 other is attached to a thick copper wire, 30 feet long, fixed to a 

 post. The vibrating fork effectually maintains the high resist- 

 ance of the receiver, except when it is affected by a spark at a 

 distance. The galvanometer is then instantly deflected, but at 

 once returns to a nearly zero position. While I was watching 

 the spot of light of the galvanometer, I noticed that it was 

 deflected when the transmitter was not in action ; after some 

 time distant thunder was heard ; in some cases the time between 

 seeing a deflection and hearing thunder was 25 seconds, a time 

 corresponding to about 5 miles ; by degrees the storm, though 

 at some distance, caused the spot of light to be deflected up to 

 25 times per minute. 



From what I noticed it is evident that signalling would be 

 somewhat seriously interfered with by a thunder-storm even at a 

 considerable distance. 



I have used the tuning-fork method of shaking the receiver 

 since last February, and have never seen it fail in its action. 

 Another method which gives good results is to mount the 

 receiver on a small projection cemented to the disc of a tele- 

 phone, in the circuit of which an electrically driven tuning-fork 

 and a battery are included. 



June 26. Frederick J. Jervis-Smith. 



Distant Cannonade. 



In answer to your note upon the distances at which Saturday's 

 salute was heard (Nature, July i, p. 204), I have to say that I 

 heard what I suppose was the salute here in Chelsea. I took it 

 at first for distant thunder, and went to look at the sky and 

 barometers. 



The newspaper reports showed, since that, that the thunder- 

 storm was much later ; and I did not hear sounds of it at all. 

 The sound reminded me rather of ^feu de j'oie than of a salute, 

 which is accounted for by the fact that the squadrons saluted in 

 succession. 



There is nothing unusual in the hearing of artillery at such a 

 distance, about sixty statute miles. The Bombay time guns and 

 salutes are often heard at the northern Mahim, a known dis- 

 tance of over fifty statute miles. They are, or were in my day, 

 very modest affairs— old-fashioned twenty-four or thirty-two- 

 pounder guns, loaded with four or five pounds of coarse black 

 powder, not all of which was burnt. I was working at warning 

 guns in that country myself for some years, and had to attend to 

 such matters, but have no notes here from which I can give 

 exact figures. 



The target practice of the forts and turret-ships at Bombay 

 was very easily distinguishable from mere salutes and time guns ; 

 not merely as a louder sound, but by being felt in the chest 

 when those could only be heard. Probably some of your naval 

 readers can tell us something of the guns and charges used at 

 Spithead on June 26. It may be presumed that they were 

 of more power than the old-fashioned artillery that I have men- 

 tioned ; but still it is probable that they were the smallest guns 

 of the fleet, and the saluting charges much less than those for 

 service. The sound produced by modern powders, too, is prob- 

 ably very different in quality from that of the old black powder 

 with which the late Prof. Tyndall made his experiments. 



The subject is of very considerable imp)ortance, and any in- 

 formation that our "up-to-date" gunners can give us will be 

 welcomed by all connected with the sea. 



102 Cheyne Walk, Chelsea. W. F. Sinclair. 



