Febkuarv, igo6.J 



KNOWLEDGE & SCIENTIFIC NEWS. 



351 



PhotogrsLpKy 



Pure and Applied. 



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



Reversal and Re-Reversal. — It was about five and 

 twenty years ago that M. Janssen observed, when photo- 

 graphing the sun, that, with the particular sensitive 

 plates he was using, an exposure of from one to two 

 hundred thousand times the suitable exposure for an 

 ordinary image gave on development a positive instead 

 of a negative, and that by increasing the exposure to 

 about a million times the image was again reversed, 

 thbugh with a considerable diminution of contrast. 

 These observations, expressed in the simplest form, 

 indicate that a gradually increasing exposure followed 

 by development, will give at first an increasing dark- 

 ness of deposit, then a diminution of density, followed 

 by another, though less marked, increase of density; and 

 there seems to be some evidence that this alternating 

 rise and fall of density resulting from a continually 

 increasing exposure, might go on until the differences 

 were too small for observation if interfering circum- 

 stances did not step in to vitiate the experiments. It 

 has further been suggested that the image utilised in 

 ordinary negative making is not the first result of the 

 action of light, but is preceded by a similar alternating 

 action to that which seems to^ follow it. If this is so 

 not only is the result of the continuous action of light of 

 an alternating character, but the maxima first increase 

 and then diminish, the effect passing, doubtless, itito 

 changes of a different character. 



It is easy to prove the increase of density with in- 

 crease of exposure as in ordinary negative making, and 

 then a decrease as in the reversal resulting from over 

 exposure, but to get further than this is, experimentally, 

 so difficult that many have tried and failed, and not a 

 few have, therefore, doubted Janssen 's original ob- 

 servations, and believe only in one increase followed by 

 one decrease in density. What appears to be a definite 

 proof of a second increase of density has lately been 

 supplied by M. Adrien Guebhard, who, in the Com pies 

 Rendus (CXLI., 559), gives a reproduction of the result 

 of one of his experiments. A Lumiere film was ex- 

 [josed to daylight for forty-four days last August and 

 .September, including only nine dull days, under a 

 graduated screen made of white paper arranged in from 

 one to twenty-two thicknes.ses, covered w'ith a cut-out 

 black paper screen. The filrri was then developed for 

 five minutes in an ordinary metoquinone developer. 

 Confirnialorv results were also obtained on Eastman 

 Kodoid films. 



The total exposure of forty-four days may seem 

 excessive in dealing with a series of only twenty-two 

 exposures, but the exposure durations ha\e to be in a 

 geometric series. An initial exposureof approximately one 

 second in a series of twentv-two, if each is double the 

 preceding, would require about the time stated for the 

 last. The impossibility of confining the light action 

 to the part required is a very great dilliculty in such an 

 experiment, because of reflection within the film. For 

 anv who wish to test the matter for themselves, and 

 reliable conlirmation of the .results obtained with an 

 extension of them as far as possible would be very 

 welcome to all interested in the study of pure photo- 

 graphy, I would make the following' observations : — 



\ pure bromide emulsion (free from iodide) gives a 

 nuirli more quickly reversible image than the ordinary 

 mixture of bromide and iodide. One considerable dis- 



advantage in the use of commercial preparations is the 

 uncertainty as to what they contain. Some plates now 

 on the market are not simple emulsions of silver salts, 

 and it seems at least probable that even the makers do 

 not know what they contain, although, of course, they 

 know what they use in their preparation. But still, 

 experiments with commercial plates are interesting and 

 may be useful. To avoid the interference of the long 

 exposures with the comparatively short exposure effects, 

 I would suggest getting the shorter exposure by means 

 of a graduating device, such as what is commonly 

 known as a .Spurge's sensitometer, and for the longer 

 exposures using separate parts of the sensitive material 

 each quite isolated from the other by being contained 

 in a separate compartment of a multiple-cell box, or one 

 of a number of small boxes. The exclusion of vitiated 

 air, the avoidance of complications due to temperature 

 changes, the results of comparative blank experiments 

 to discover any interfering circumstances that may be 

 present, the distinction in the result between printing- 

 out and development effects, and other sucn matters 

 must, of course, be taken into consideration. 



Shading the Lens. — The importance of so shading the 

 lens, especially when working out of doors, that ex- 

 traneous light is excluded from the camera, is not 

 recognised now as it used to be, and never was 

 generally recognised as it ought to be. If the glass of 

 the lens were perfectly transparent, its surfaces per- 

 fectly polished, the interior of tne mount of the camera 

 perfectly non-reflecting, and the air perfectly free from 

 motes, there would be, perhaps, no advantage in 

 shading the lens ; but such conditions are impossible. 

 The hood generally found on old lenses, but often not on 

 new ones, is a little better than nothing. Years ago 

 some photographers made large conical hoods, almost 

 reminding one of small gramophone trumpets, and 

 these were used even in studios with advantage; but 

 this is not the kind of thing that will best serve the 

 purpose. It is obvious that the shape of the opening 

 at the outer end of a large hood or shade should corre- 

 spond to the shape of the plate or that part of the plate 

 that is to receive the desired image. If a light bellows 

 could be attached to the camera front so that it could 

 be opened out to extend a few inches in front of the 

 lens, and carry at its outer extremity a screen with an 

 adjustable rectangular opening in it, the shading of 

 the lens would be ideal. This principle, but without 

 the possibility of adjustment, may be easily carried out 

 in those hand cameras in which the lens is within an 

 outer case. The opening in the front of the case may 

 be so made that w^hile the plate is fully illuminated, all 

 the light that can be safely cut off is prevented from 

 entering the camera. 



There is also advantage in using a camera made to 

 carry a larger phite than that used and in fitting lenses 

 into mounts of a larger diameter than is necessary just 

 to hold them, ;ls in both cases the reflecting walls are 

 further away from the path of the light being utilised, 

 and the rellection is reduced. In spectroscopy and 

 photo-micrography these matters are of considerable 

 practical importance. 



The O plical L'rojeclion of Opaque Objects. — A piece of 

 apparatus that deserves to be more widely known is 

 the " Picture Postcard Lantern," made by Messrs. 

 \\'. C. Hughes and Co., of Mortimer Road, Kingsland. 

 It consists of a well-made and capacious Russian-iron 

 lantern, within which arc two incandescent gas burners, 

 one on each side, that illuminate the card held in a 

 carrier at the back, and a lens in the front of the 

 lantern that projects the image on to a small screen 

 three or four feet away. While originally meant for 



