Sept., 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



231 



P hotography . 



Pure and Applied. 



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



The Developable Image. — Professor J. Joly, in his 

 presidential address to the Photographic Convention, 

 deals with the nature of the developable imag:e. He con- 

 siders the change to be of a physical nature, and with 

 due reserve suggests photo-ionisation as the cause, 

 admitting, at the same time, that he has not as yet 

 been able to detect any electronic discharge from the 

 film under light stimulus. I should like to ask whether 

 the wonderful persistence of the developable condition 

 is not a considerable difficulty in the way of accepting 

 such a theory. Whether or not it is supposed that the 

 almost inconceivably minute stimulus that we know to 

 be sufficient to produce the developable condition does 

 so by effecting a change in the electrical condition of 

 the salt, it must be remembered that the changed state 

 of the salt is able to persist for years in an aqueous and 

 salt-containing medium, that is, without insulation, and 

 in the case of the Daguerreotype on the surface of the 

 best known conductor. As Professor Joly says, "our 

 knowledge of the electron as an entity taking part in 

 many physical and chemical effects, should be kept in 

 sight in seeking an explanation of the mode of origin of 

 the latest image," and it is, I submit, of even greater 

 importance to be guided by known facts and experi- 

 mental data, and to go forward in our conceptions only 

 as these justify our progress. lonisation may serve well 

 as a working hypothesis, whether or not the future will 

 prove, but I think it should not be accepted even as a 

 possible theory of the nature of the developable image 

 until some definite experimental support can be shown 

 in favour of it. 



Measuring Vessels. — I suppose that it is correct to 

 regard weights and measures simply as conveniences, 

 and to value all arguments put forward in favour of this 

 or that system by comparing them from the same point 

 of view. The superior convenience of one system over 

 another may be the merchant's, or it may be his 

 customer's, and then the man of business has to en- 

 deavour to find the value of the respective conveniences 

 that he may follow the more profitable course. The 

 practical photographer is not concerned with profits in 

 this matter, but only with minimising his own trouble. 

 Unless one is already more accustomed to the metric 

 system, there can be no doubt that, at present, the 

 ordinary English weights and measures are more con- 

 venient for English people, for all English formula are 

 so expressed. But among all the arguments set forth 

 in favour of either our present methods or the annihila- 

 tion of them in favour of the metric system, there is 

 one very practical matter that I do not remember having 

 seen emphasized as it deserves to be, namely, the 

 shapes of the measures in common use. Whether one 

 purchases a two-dram, two-ounce, four-ounce, pint, or 

 quart measure, it is almost always of a convenient 

 shape, but measures on the metric system are tubular. 

 Of course, a narrow tube is better adapted for exact 

 subdivision, but exactness is not the primary desidera- 

 tum ol the practical photographer — an error of a few 

 per cents, on either side of the true capacity is negligi- 

 ble because the effect of the difference is rarely 

 recognisable. The photographer wants convenient 

 vessels for pouring from and into, when a flat dish is 



the other receptical. I think this simple but very 

 practical matter well worth the serious attention of 

 those reformers who are seeking to get the metric 

 system universally adopted. 



Fine Grained Images. — Messrs. Lumiere and Seyewetz 

 find that a finer deposit than otherwise is obtained by 

 developing slowly (by adding either water or a re- 

 strainer) in the presence of a solvent of silver bromide. 

 For this latter they use from 15 to 20 grams of 

 ammonium bromide to each 100 cc. of developer. 

 Paraphenylene-diamine and orthoamidophenol need no 

 such addition, as developing solutions prepared with 

 them have the necessary solvent power. W'orking on 

 such lines will probably be found to incur risks not 

 usually met with. Silver in solution is liable to give 

 stains, as with ammonia developers that were generally 

 used before soda developers became so common. I 

 think that the almost universal use of sodium carbonate 

 instead of ammonia is a case of the survival of the 

 fittest, and that it would not be well to go back to 

 ammonia with all its uncertainties. A fine grained 

 image is not everything, and the old wet collodion 

 plate, which is often taken as the standard, if developed 

 with ferrous sulphate, gave a coarse grain, though the 

 particles were more uniform in size than is generally 

 the case in gelatine plates. It is not so much the 

 coarseness of grain in gelatine plates that causes 

 trouble, as the presence of a comparatively small pro- 

 portion of large grains, some of which appear to be 

 often due to imperfect filtration of the emulsion, for 

 they settle down to the lower side of the film. But 

 granting that the proposed methods are not the best 

 for general adoption, cases may arise where they will 

 be serviceable, and it is very desirable to know the 

 characteristic effect of any possible procedure. In the 

 presence of the solvent of the silver salt, it is supposed 

 (.-md doubtless it is a fact) that some of the silver that 

 forms the image is deposited from solution, a kind of 

 intensification effect, the other part being reduced, as 

 usual, from the solid particles of salt as contained in 

 the emulsion. It would be interesting to know what 

 effect, if any, the double origin of the developed image 

 has on the gradation. 



licccjvcd. — J. H. Dallmeyer, Ltd., send a catalogue 

 of their well-known lenses and other specialities. The 

 frontispiece shows the usefulness of the "Adon," which, 

 although a small lens intended for attachment to hand 

 cameras, has here, used alone, given an excellent 

 12 by 10 photograph with a camera extension of 38 

 inches. 



REVIEWS OF BOOKS. 



The Preparation and Mounting of Microscopic Objects, by T. 



Davies (C. Arthur f'earson ; fcap. 8vo, pp. iiS; js.). This 



is a reprint of a book which has had a large sale in past days ; 

 and, in spite of certain faults of arrangement, not only was 

 well worth reprinting, but deserved to be reprinted in better 

 style than the " edition " now before us. The book is mani- 

 festly merely a new impression from the old stereotype plates 

 of 1873, though this is not mentioned; the binding and the 

 paper have alone been altered — the latter very much for the 

 worse. In fact, the paper is both thick and coarse, and quite 

 unsuitable for the purpose, though the price at which the book 

 is published would surely have justified more satisfactory 

 treatment in this respect. The book itself is too well known 

 to need criticism. It was, of course, written entirely for the 

 amateur ; and, though somewhat out of date now, contains 

 much information on preparing and mounting objects for the 

 microscope which is of real service. — F.S.S. 



