30 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Feb., 1905. 



Photography. 



Pure and Applied. 



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



Dr. Russell's Experiiiie>its. — Ln^t month I referred to 

 the production of the developable condition by emana- 

 tions from various substances as in Dr. Russell's ex- 

 periments, and showed that although some results 

 appear to indicate that the effect is due to a gas, and 

 that this gas is the vapour of hydrogen peroxide, others 

 are generally allowed to be difficult to account for on 

 this simple hypothesis. 



Professor j. Joly, in a letter to Xature last August, 

 asks with regard to Dr. Russell's experiments and his 

 suggestion that peroxide of hydrogen is the active 

 agent, " ought we not rather to seek the explanation in 

 the ionising properties of metals indicated by other 

 observations?" He founds this question on his ob- 

 servation that pure mercury and polished speculum 

 metal in contact with a rapid plate under absolute 

 alcohol in an airtight desiccator over calcium chloride 

 produced the developable condition in a gelatino- 

 bromide plate just as vigorously as if it were obtained 

 in ordinary moist air. It is, perhaps, worth while to 

 sum up the recorded experiences of Dr. Russell with 

 regard to conditions similar to those described by 

 Professor Joly. 



.\s to mercury. Dr. Russell found that, if pure, it was 

 inactive, and that an active sample might be made in- 

 active by purification, and that if pure and inactive the 

 addition to it of one-thirty-thousandth of its weight of 

 zinc rendered it very active. 



As to moisture. Dr. Russell at first found no differ- 

 ence whether the action took place in air saturated with 

 moisture or air dried with sulphuric acid or calcium 

 chloride, or in an atmosphere of hydrogen. Later on, 

 when he had found how intimately hydrogen peroxide 

 was connected with the results obtained, he remarks 

 that a gelatine plate is never really dry. He seems to 

 have repeated the experiment of trying the different 

 effects of an atmosphere dried by calcium chloride and 

 an atmosphere kept moist, and found that after three 

 days " the damp plate had much the darker picture on 

 it." By passing dry air over zinc nothing was given 

 off that affected a plate, but when moist air was passed 

 over the metal and then allowed to impinge on the 

 plate, the change was effected. He found also that 

 dry alcohol neither transmitted the action nor was 

 made active by putting zinc in it, but by adding the 

 merest trace of w-ater to the alcohol, the zinc did make 

 it active. 



Messrs. Blaas and Czermak (Science Abstracts, 

 Section A, No. 2559, 1904) record the old and well- 

 known fact that many substances after exposure to 

 light are able to affect a photographic plate, or affect it 

 more readily than they did before insolation. They 

 say that this property is connected with the occlusion 

 of ozone and that bright or amalgamated zinc possesses 

 the property, and that many substances emit a diffuse 

 radiation which is reflected at mirror-like surfaces. 

 Dr. Russell states that he found ozone to be without 

 effect, and in a communication to the Royal Society last 

 June dealt with the effect of exposure to sunlight in 

 rendering " active " substances more active. He says 

 that " bodies other than those which may contain resin 

 or allif^d '^nh<;tnnres are not affcrtrrl in fhi<; w.nv by 



light." " Metals are not rendered active by sunlight." 

 Dr. Luppo-Cramer finds that while a gelatino- 

 bromide plate is affected by hydrogen peroxide, a coUo- 

 dio-bromide plate is not, and hence considers that the 

 gelatine has a vital influence on the result. Hut his 

 experiments are not strictly comparable with Dr. 

 Russell's, as he immersed his plates in weak .solutions 

 of the peroxide, though he considers that this is the 

 same in effect as exposing them to its vapour. I be- 

 lieve it has been observed that films apart from the 

 glass support are not affected by these emanations. 

 Thus it would appear that the glass, which is not 

 permeable by them, is necessary to prevent them pass- 

 ing through the gelatine film and escaping with the 

 production of little or no effect on the sensitive .salt. 



There arc many other observations that bear upon 

 this subject in a more or less direct manner, but I 

 think that I have set down sufiicient to show that there 

 remains a considerable measure of uncertainty with re- 

 gard to some of the observations, and that it is im- 

 possible to rest satisfied with the suggestion that the 

 effects are the simple results of the action of (leroxide 

 of hydrogen. The peroxide doubtless has something 

 to do with it, and, perhaps, is itself one of the effects 

 of the action rather than the cause. We do not yet 

 know how silver bromide is changed when it assumes 

 the developable condition, though the evidence is very 

 strongly in favour of a merely physical alteration. 

 Such an alteration seems more likely to result from the 

 impact of some form of radiant energy, than to be the 

 direct result of mere contact with such a suljstancc as 

 peroxide of hydrogen. 



T/ic Use of the Optical Zaw/ir;/. — Projection lanterns 

 are often used in such a manner that one might 

 well suppose that they are regarded simply as 

 " magic " lanterns, and that so long as an enlarged 

 image of the slide is produced on the screen, and that 

 the image is tolerably well defined and sufficiently 

 bright, every desirable condition has been fulfilled. 

 .Some, though not many, go so far as to consider the 

 convenience of the audience and endeavour to arrange 

 so that the middle of the screen is, al the highest, about 

 level with their eyes. .Still fewer pay attention to what 

 should be one of the simplest and most primary of 

 rules, namely that the image on the sheet ought never 

 to be seen to move or vary in any way in the matter of 

 adjustment. But there is very much more than this in 

 the correct use of a lantern. There is a proper point 

 from which every flat representation of a solid object or 

 \iew should be looked at, and the skilful or scientific 

 exhibition of a picture renders it at least possible for the 

 observer to see it from this point or from a position at 

 a similar or greater distance. If a three-inch slide is 

 printed by contact from a negative taken with a six- 

 inch lens, the viewing point is always equal to twice 

 the length that the full three inches would be repre- 

 sented by r)n the sheet. If a twelve-foot sheet would 

 be covered by the three-inch slide, the nearest specta- 

 tor should be twenty-four feet from the sheet, t'nder 

 these conditions the view as shown will subtend the 

 same angle (or a less angle, allowing for the spectators 

 who are behind the front row-) as the original view did 

 from the position at which it was photographed. But 

 suppose the hrdi is not large enough to allow- of such a 

 distance, some may object. If the distance between 

 the front row and the sheet cannot be more than twelve 

 feet, then bring the lantern nearer and give a six-foot 

 picture, and the conditions are fulfilled. Of course, 

 size counts for something, but mere size, mere 

 exaggeration is contemptible. 



