230 



DISCOVERY 



struction of sheds, and the insurance of personnel, 

 airships, and installations. British firms have also 

 offered to assist in buUding the airship under the 

 super\dsion of and in accordance with the plans of the 

 Zeppelin engineers, using British workshops and sheds. 

 An American firm is also negotiating for the organisa- 

 tion of an airship service between Buenos Ajtcs and 

 Chicago. 



The Secret of the 

 Photographic Plate 



By T. Thorne Baker 



One of the most fascinating problems of modern 

 physical chemistry is undoubtedly that of the action 

 of light on the photographic plate. We know that 

 an image cast by a lens upon a sensitive plate for a 

 twenty-thousandth part of a second can produce an 

 invisible change in the film, known as the " latent 

 image," which at once, or years afterwards, can be 

 converted into a negative image by the action of a 

 suitable reducing agent. 



We know, too, that there are very slow plates, used 

 in the making of process negatives for half-tone repro- 

 duction, on the one hand, and other plates of the 

 most extraordinary rapidity on the other, which are 

 all made with one common sensitive substance, silver 

 bromide. 



What is the difference between the slow plate and the 

 plate fifty times as sensitive ? UTiat is the cause of 

 sensitiveness, and what limits the ultimate sensitiveness 

 to be attained ? These are questions to which, ob- 

 viously, the research chemist of the plate factory devotes 

 most of his attention, but during the past five years the 

 British Photographic Research Association has con- 

 centrated a great deal of valuable work on the subject, 

 and quite recently some really definite information has 

 been got together and made available. 



Photographing Negatives with the 

 Microscope 



If a photographic negative be examined under the 

 microscope, the image will be seen to consist of in- 

 numerable fine " grains " of reduced silver ; but if a 

 small fragment of the undeveloped film be dissolved in 

 water, and examined with a high power, it will be found 

 to consist of considerably smaller grains, or actually 

 crystals, of silver bromide. The secret of the photo- 

 graphic plate lies in these crystals, their formation, 

 growth, and physico-chemical treatment. A great 



deal of excellent work has been done during the last 

 two or three years in the refinement of the technique 

 of photographing them with the microscope. B}' using 

 a Pointolite lamp as light source, and preparing films 

 one layer thick only in silver bromide crystals, images 

 of the latter, having a magnification of as much as 

 3,000 diameters, can be projected with yV-in. oil- 

 immersion objective upon a drawing-board several feet 

 away. The apparatus must be mounted on a vibra- 

 tion-free table, and with a little practice perfectly 

 satisfactory photogiaphs of the crystals can be obtained. 

 The crystals are first of all focused upon a piece of 

 white paper, and after a red light filter has been placed 

 in the optic axis at some convenient spot, a plate is 

 pinned to the drawing-board in the desired position, 

 and the exposure made by removing and replacing 

 the light filter. 



When the silver bromide is first obtained — as a 

 precipitate formed by mixing ammonium bromide 

 solution with silver nitrate solution in the presence 

 of gelatin — the crystals are so small that all attempts 

 so far to resolve them have failed, and they therefore 

 appear as spherical particles. In this preliminary form 

 the silver bromide is in an extremely insensitive state, 

 so much so that it may safely be exposed to weak 

 artificial light without danger of " fogging." Hereafter 

 follows the ripening, during which the particles gradu- 

 ally evince their crystalline shape and incidentally 

 gain in sensitiveness manj' thousands of times. 



Fast and Slow Plates 



The crystals are not all of the same size, but may 

 vary very considerably. It would appear that the 

 larger crystals can actually absorb smaller ones and 

 grow at their expense, though this is by no means 

 proved. The character of the plate, however, depends 

 largely upon the general character of the crystals. 

 Thus if their size be fairly regular, the resulting plate 

 will be vigorous in' character, and give great contrast 

 such as is required by the process or photo-mechanical 

 plate. The uniformity in size will produce this type 

 of plate even though the crystals may be of the 

 large t3^e usually associated with fast plates. Certain 

 it is that in the ripening or digesting process the 

 crystals grow to a far larger size, and that it may be 

 said in general that the crystals in a fast plate are 

 always larger than those in a slow one. 



The plate \\hich gives good gradation has been 

 found to obtain a very mixed selection of crystals, 

 the diameters probably varying as much as twenty 

 times. Large crystals have been grown on recognised 

 lines which have exhibited very beautiful patterns, 

 and might possibly possess enormous sensitiveness, 

 but their size would preclude them from being used 

 in a commercial plate. The published work of many 



