January, igo6.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



329 



PhotogroLpKy 



Pure and Applied. 



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



Is Vevelopmcnl a Reversible Reaction? — It seems that 

 my remarks under this heading in the last number did 

 not make clear, except, perhaps, in a negative sense, 

 what I consider the word reversible should mean in its 

 application to development. " Many chemical pro- 

 cesses are reciprocating-, i.e., the original substances 

 may be re-obtained from the products of the reaction." 

 Such changes are termed reversible. In a purely 

 chemical sense, we may be satisfied with the chemical 

 identity of the original and the reproduced substances, 

 but if development as distinguished from mere chemical 

 reduction is reversible, the original and the reproduced 

 substances must be photographically as well as chemi- 

 cally identical. To be still more explicit I suggest the 

 following experiment : — Let a plate be subjected to a 

 graduated exposure, leaving a part unexposed, and let 

 it be treated as follows in the dark, removing a sample 

 strip at each stage for subsequent critical examination : 

 (i) Develop it. After simple washing (not fixing), (2) 

 treat it with a solution of the products of the complete 

 oxidation of the developer used. This should lestore the 

 plate to its condition before development. {3) Treat it 

 a second time with the developer. If development is 

 a reversible reaction the image will again be de- 

 veloped up as at tirst. The unexposed silver bromide 

 is still present, so that in the second development a 

 definite distinction is made betw'een developable and 

 non-developable silver salt. 



If development is a reversible change it may be ex- 

 pected that very interesting results will follow. Per- 

 haps one of the most important of these will be the 

 indication that the change produced by light (that is 

 the difference between de\elopable and non-developable 

 silver salt) is independent of the halogen. The 

 possibility of changing a silver .salt that has been made 

 developable by exposure into another silver salt, with- 

 out losing its developable property, a fact long since 

 known, may be considered as pointing in the same 

 direction. And the case now supposed would go a step 

 further, the halogen being renio\ed entirely and after- 

 wards restored to the metal. Experiments seem to prove 

 that the developable condition does not depend i)n the 

 medium that supports the silver salt. If the change is 

 in neither the halogen nor the medium, we .seem almost 

 driven to the conclusion that it is in the silver. And 

 this reminds us of Cleneral Waterhouse's experiments, 

 in \\ hich he got the developable condition on a polished 

 silver surface, though I belie\e he failed w hen the silver 

 was cleaned by treating it with acid and heating it. 



Superposed Slereoscopic Prints.— Those stereoscopic 

 pictures supposed to be made by printing the one mem- 

 ber in red and the other in gre<_-n, the one over the 

 other, and that are \ iewed by spectacles having a red 



and a green glass, one for each eye, ha\e nearly always 

 bothered me, and, I suppose, other people, by struggles 

 between stereoscopic and pseudoscopic effects, and, 

 apparently, other confusions, in some parts of them. 

 Mr. Alexander Thurburn, of Keith, Banffshire, in con- 

 nection with this subject, has recently sent me two 

 stereoscopic diagrams of a pyramid in red and green 

 lines, but in the one the lines are on a white ground 

 and in the other on a black ground. Although the 

 disposition of the coloured lines is the same in both, 

 everyone who looks at them through the coloured 

 spectacles finds the one pyramid to project and the other 

 to recede. By turning the card on a horizontal axis 

 the apexes appear distinctly to move in opposite direc- 

 tions, as they would in the c:ise of a solid and a hollow 

 pyramid. The question was, Is this difference the 

 effect of the difference in the background, and if so, 

 why ? I soon found that on the dark ground the lines 

 seen through either coloured glass were those of the 

 same colour as the glass, but on the w-hite ground only 

 the green lines were seen through the red glass and 

 the red lines through the green glass. It is clear, 

 therefore, that as the coloured lines are similarly dis- 

 posed in the two diagrams the one must appear stereo- 

 scopic and the other pseudoscopic. The reason is clear. 

 The red lines seen through the red glass appear white 

 and are, therefore, lost on a white ground but visible 

 on the black. Through the green glass the red lines 

 are apparently black, and, therefore, they show on the 

 white but n;)t on the black ground. The green lines 

 are similarly affected, but in the reverse sense. 



In the case of the stereoscopic pictures referred to we 

 have to consider red and green, and the white surface 

 these are printed on. Through the red eye will be seen 

 the green if surrounded by either red or w hite, and also 

 the red where it is surrounded by green. Through the 

 green eye will be seen the red when surrounded by 

 either green or white, and the green when surrounded 

 by red. .AlS each eye sees, or may see, parts of both 

 the red and the green pictures and cannot see other 

 parts, if each of the stereoscopic pair is printed in a 

 single colour it seems that pseudoscopic and stereo- 

 scopic effects must be mixed and confusion result, unless 

 the subject is a very exceptional one. Mr. Thurburn 

 has been good enough to send for my inspection a 

 number of the " Plastische Weltbilder," in which this 

 confusion seemed to him to l>e absent. I find it is 

 also absent to me. On careful examination I find that 

 the pictures for both eyes are printed in both colours — 

 that is, each eye, using the coloured spectacles, sees 

 parts of both red and green, but not the same parts. 

 The distribution of colour is systematic throughout, 

 and a conspicuous example will show the character of 

 it. A shop front in the foreground has white sash 

 bars in front of a dark interior, and over the front the 

 name is in dark letters on a light ground. The picture 

 provided for the one eye and the picture that is really 

 seen by that eye, has green sash bars and red letters. 

 The picture for the other eye has red sash bars and 

 green letters. The sash bars are light on a dark back- 

 ground, and the letters are dark on a light background. 

 Therefore the representation of objects bv such means 

 is not a simple case of printing' the one picture in red 

 and the other in green, and discriminating them bv 

 means of spectacles with one red and one green eve. 

 This simple and straightforward method must, I think, 

 be faulty, and success appears to depend on a distribu- 

 tion of colour by hand. If it is so, these productions 

 cannot rightly claim to Ix.' photographs, and the method 

 is not useful for scientific purposes, as it might other- 

 \\ ise be. 



