300 



KNOWLEDGE & SCIENTIFIC NEWS. 



[December, 1905. 



PhotogrsLphy 



Pure and Applied. 



By Chaf.man Jones, F.I.C, F.C.S., &c. 



Distortion with Focal-Plane Shutters. — It has often 

 been pointed out that an exposure made by passing a 

 slit of>ening over the face of the sensitive plate must 

 lead to a distorted image of a moving object, because 

 the various strips of the object corresponding to the 

 slit opening are photographed consecutively as the ob- 

 ject moves. ThLs is so obviously the case that it is 

 surprising to find the statement met with answers to 

 the effect that if distortion exists it is negligible because 

 it is rarely discernible. Whether or not distortion is 

 visible must depend very largely upon the eye of the 

 person who looks for it, and upon the character of the 

 object distorted. A critically-trained eye can see what 

 ordinary observers never will be able to see, and there 

 are some distortions that cannot be detected unless the 

 object, or an undistorted image of it, is available for 

 comparison. A short, broad person, for example, may 

 be improved in appearance by having his height in- 

 creased or his breadth diminished by twenty per cent. 



Some excellent examples of distortion with focal- 

 plane shutters will be found in the current volume of 

 the "British Journal of Photography" at page 807 et seq. 

 Here may be seen motor cars all out of shape, and 

 photographs of a rotating white strip or lath which do 

 not suggest the object photographed at all. .At page 

 858, Mr. C. Welborne Piper gives two photographs of 

 a riding bicyclist. In one the man is slight and the 

 wheel-base of his machine short, in the other the man 

 is stout and the wheel-base long, and the wheels are 

 distorted in both. It must not be concluded from these 

 examples that focal-plane shutters are useless instru- 

 ments, but rather that they should always be avoided 

 when a shutter at the lens diaphragm, or as near to it 

 as possible, will do the work. It is certainly true that 

 with a focal-plane shutter the lens acts with practically 

 its full aperture all the time, but the advantage of this 

 has been very much over-rated. Many who lay .so 

 much stress on it probably never knew the rate of 

 movement or the equivalent exposure given by any of 

 their shutters in any of their exposures. A record of a 

 movement that requires an exposure of less than the 

 two hundredth or three hundredth of a second can often 

 be obtained only by means of a focal-plane shutter. 

 The resulting distorted image may l)c sufficient record 

 as it stands, or it may be possible to eliminate the dis- 

 tortion by redrawing it, if the constants of the shutter 

 are known. 



Is Development a Reversible Reaction? — In the October 

 number of this journal I referred to a paper on this 

 subject by Mr. S. H. Shcppard, published in the 

 Chemical .Society's Journal for August, and remarked 

 that in using the word " reversible " the author does 

 not appear to consider the difference between de\elop- 

 able and non-developable silver bromide. I said " the 

 silver bromide is reduced by the developer because it is 

 in the developable condition, undevelopable silver 

 bromide not being reduced under the same conditions. 

 When the reaction is reversed, the resulting silver 

 bromide would, I suppose, not be likely to be in the 

 developable state." In answer to this Mr. Sheppard 

 writes as follows. — 



" In the October issue of your journal, Mr. Chapman joncs, in 

 referring to a paper of mine on the reversibility of photographic 



development (Journ. Chem. Sov. — Sept. 1905) criticises the applica- 

 tion of the term ' reversible ' to this reaction on the ground that 

 the silver bromide reduced by tlie developer is in a ' developable' 

 condition, due to the action of light, whilst that reformed by the 

 reverse reaction to development is presumably not so. The phrase 

 ■ developable silver bromide ' can hardly l)e said to express any 

 exact conception, since with a reduciiv.; agent of sufficient potential 

 silver bromide, either alone or emulsitied, can be reduced to silver 

 without any previous exposure. If. however, we take it to mean in 

 this case, reduction by the ordinarv developing agents of practice, 

 Mr. Jones' criticism still fails to hold, ;is the silver bromide 

 reformed by the re\erte ' bleaching ' action is • developable ' 

 forthwith and without any preliminary exposure to light, a fact in 

 agreement with photographic practice in intensification by 

 redevelopment (r/. Messrs. C. W. Piper and D. J. Carnegie, 

 Amateur Photograj'hcr ior June, 1905). Thereat point at issue is, 

 of course, the nature of the so-called ' latent image ' formed by 

 light, but this question, however interesting, cannot be dealt with 

 in a brief letter." 



It has long been known that the reaction between 

 ferrous t)xalate and silver bromide is rcxersible. For 

 more than twenty years practical advantage has been 

 taken of this fact in the use of ferric oxalate to thin or 

 dissolve away a part of the silver from a negative that 

 is too dense. It may be objected that no alkaline 

 bromide is added in this case, but that does not affect 

 the essence of the reaction. Mr. Shcppard has deter- 

 mined the conditions of equilibrium, including the 

 effects of alkaline bromide and dilution. In doing this 

 he claims to have " shown experimentally that develop- 

 ment is a reversible chemical reaction " (quoted from 

 his abstract of his paper in the Proceedings of the 

 Chemical .Society). 



I cannot see that he has done anything towards 

 proving development to be a reversible reaction. De- 

 velopment and simple reduction of the silver salt must 

 l>e distinguished. A reagent that will reduce silver 

 bromide will not, therefore, develop an image. .Silver 

 bromide exists in the developable and the non-dovclop- 

 able conditions, although it is reducible to the metal in 

 both conditions. Mr. .Sheppard says that " ' de\elop- 

 ablc silver bromide ' can hardly be said to express any 

 exact conception," but this ha.s nothing to do with the 

 matter. We are dealing not with conceptions hut with 

 facts. The difference between developable and non- 

 developable silver bromide is a fact, it is more than the 

 chief corner stone of photography, it is the very fotnida- 

 tion of it, and conceptions, exact or otherwise, do not 

 affect it. It is just in this that the difference exists 

 between simple reduction from a chemical point of 

 view and development from a photographic point of 

 view, and so far as I can see Mr. .She])i)ard h;is not 

 gone beyond the simple chemistry. 



But suppose for a moment that Mr. .Shcppard h.id 

 pro\ed development with ferrous oxalate to be a re- 

 \ersible change, he would not have proved development 

 in general to be reversible, because ferrous oxalate is 

 a developer by itself, and, as a matter of practical fact, 

 is now very rarely u.sed. There arc many developers of 

 quite a different character (chemically considered) and, 

 perhaps, different in their action as develojx'rs. 

 Probabl)* the most simple f)f these is hydroquinone (or 

 quinol). Of this Mr. Sheppard him.self says " this re- 

 verse reaction is largely nullified by the presence of 

 alkali and alkali sulphite, alw;iys used with organic 

 developers, as the.se substances alone or mixed react 

 with the quinone, reducing it to quinol." So there 

 seems to be a difficulty here, even with so simple a 

 substance as hydroquinone, in proving dcvelopnuwit to 

 1h; reversible — a difficulty th.-it Mr. Sh<pp;ird h.is not 

 overcome. And even if development with both ferrous 

 oxalate and hydroquinone were proved to be reversible 

 changes, it would not necessarily follow that develop- 



