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KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1906. 



Photography. 



Pure and Applied. 



By Chapman Jones, F.I.C, F.C.S., &c. 



The Developable Image. — During the last few months 

 there has been a good deal pubHshed about the develop- 

 able image. The speculations as to the character of 

 the change wrought in the siher salt when it is converted 

 from the non-developable to the developable modifica- 

 tion may, as always before, be divided into two classes, 

 namely : (i) those in which decomposition of the silver 

 salt is assumed, and (2) those in which it is taken for 

 granted that the salt is not decomposed. Some of the 

 more recent ideas can claim no other novelty than that 

 they axe expressed in more modern terms, for with so 

 simple a substance as silver bromide there is not very 

 much scope for ingenuity in devising suggestions as to 

 its possible changes. When a chemical change, that 

 is, decomposition, is assumed, it is considered that 

 there may be formed a solid solution of a silver bromide 

 that contains less bromine than .\gBr, or, perhaps, of 

 metallic silver, in the unchanged bromide. Assuming 

 that there is no decomposition of the molecule, it is 

 suggested that the change may be due to the loss of an 

 electron. 



The one certainty that underlies all these speculations 

 is that there is a change. I think that we may go a 

 step further and state that the change is of a physical 

 kind, because I believe that it is impossible to conceive 

 of a change that is not physical. But as to whether 

 the change goes a stage further and actual decomposi- 

 tion results, or if the change is physical only, the 

 nature of it, are problems that appear to be but little 

 affected by recent publications. It is easy to say that 

 certain theories are in accord with certain experimental 

 results, but the difficulty is to find a theory that will fit 

 all the facts. A catalogue of the known properties 

 of developable silver bromide would be of much more 

 use than a great many of the suggestions as to its con- 

 stitution, and the list would certainly show that several 

 of these suggestions cannot be true. If such a cata- 

 logue were made, some idea of the character of the 

 evidence upon which each of the supposed properties 

 rests should be given, for some investigators ha\e a 

 most unfortunate way of mixing up their theoretical 

 ideas with their experimental results. 



Precision in Photography. — I suppose that there is no 

 other art that is practised in so happy-go-lucky a way 

 as photography, even when taken up by those who 

 have been trained in accurate manipulation. The pre- 

 cision wanted is not in compounding developers to 

 within a unit or two per cent., nor in timing develop- 

 ment to a second, this would be foolishness and not 

 exactness, but it is surprising, for example, how rare 

 is the ability to focus critically. In the majority of 

 cases it is not due to defective vision, but merely to a 

 careless want of appreciation of what critical focussing 

 is. Perhaps the inherent inexactness of work done bv 

 holding a camera in the hand has something to do with 

 it, but the chief reason appears to be that the attitude 

 of even scientific persons towards photography is often 

 strictly comparable to that of the very ignorant with 

 regard to microscopy, whose notion is that anyone can 

 look at an object through a microscope and see an en- 

 larged representation of it. We have improved a little 

 in this during the last few years, but not very much. 

 The successful practice of any art requires a sympathetic 

 appreciation of its possibilities, and a clear understand- 



ing of the requirements of each particular application 

 of it. 



Uranium Toned Prints. — Uranium toned bromide 

 prints are notoriously so unstable that those who 

 desire the warm colour they show would do well to try 

 toning with copper ferricyanide instead. It is curious 

 that a uranium intensified negative has the reputation 

 of stability, and appears to deserve it, though there is no 

 sullicient reason to doubt that the change is the same 

 in both cases. M. L. Lemaire states that he has 

 traced the liability to change in prints, to the presence 

 of silver ferrocyanide, and says that if this is removed 

 they become stable. He proposes to remove it with 

 potassium thiocyanate, or oy treating the prints with 

 a weak alkali followed by dilute nitric acid. This last 

 method must present difficulties, as alkaline solutions 

 will remove the uranium compound. .Mthough the fact 

 is undisputed that uranium toned prints are very liable 

 to fade, the reason is still not clear. Indeed, the exact 

 change that takes place during toning and the com- 

 position of the resulting image do not appear to have 

 been determined. 



Received. — Messrs. Ross, Ltd., send their catalogue 

 for igo6, which includes their well-known lenses, several 

 between-lens shutters, including a new one, the 

 " Koilos," and cameras, &c. 



Mr. Butler, of 20, Crosby Road, Birkdale, Southport, 

 sends a new illustrated pamphlet of his patent " swin- 

 cam " camera stand. 



' ' What can be done with a Goerz lens ? " is an ex- 

 cellently illustrated pamphlet, from C. P. Goerz, i, 

 Holborn Circus, that describes the various uses and 

 capabilities of the several kinds of photographic objec- 

 tives made by this firm. 



A copy of either of the above will be sent on applica- 

 tion to the firm concerned. 



Correspondence. — H. E. McColl. — The process of 

 i-hlorinating the developed but unfixed plate, dissolving 

 nut the silver chloride with ammonia and then reducing 

 the silver bromide, does give a fogged image for the 

 very reason you suggest. Probably the seeming dis- 

 crepancy between the amount of bromide left in the 

 high lights and the fogging is apparent only, it is diffi- 

 cult to judge by the mere appearance. You might try 

 the effect of omitting the ammonia and communicate 

 the result if of interest. It is not possible to say any- 

 thing about your second experiment as the composi- 

 tion of the film you started w'ith is not known except to 

 the makers. The conversion of P.O. P. into a develop- 

 ment paper is not new. 



Papers R-ead. 



At the meeting of the Zoological Society held on March 6, 

 Mr. R. Shelford read a note on flying snakes alluded to else- 

 where; the other communications at the same meeting con- 

 sisted of reports on the organisms obtained from Lake Tan- 

 gan3ika and other Central .African lakes during a recent 

 expedition. Of these, Mr. Boulenger took charge of the 

 fishes. Dr. Caiman reported on the crustaceans, Mr. Edgar 

 Smith discussed the mollusks, and .Mr. Kirkpatrick de- 

 scribed and recorded the sponges. .At the meeting of the 

 same .Society held on March 20, Mr. Thomas described a 

 new form of brown bear from the Shan .States of L'pper 

 Burma, which is of special interest from the fact that bears 

 of this group have hitherto been quite unknown from this 

 district. Mr. G. A. K. Marshall, on the same occasion 

 described no less than 22 species of beetles belonging to the 

 genus Sciobius ; while Dr. Gadow discussed the evolution 

 of the colour-pattern and the arrangement of the scales in 

 the Mexican lizards of the genus Cnemiiidphoriis. .\t the 

 meeting of the Geological Society on March 29, Mr. S. S. 

 Buckman discussed " homoemorphy " in brachiopods. 



