June 23, 1882.] 



KNO\VLEDGE 



55 



Other instances of the same sort may readily be cited. 

 A very noteworthy case, in which the imperfection of 

 ordinary vision and ordinary methods of delineation caused 

 many men of science to be long led astray, till photography 

 finally came to their rescue, is that of the solar corona. The 

 pictures of the corona whicli used to be drawn by different 

 observers of the same eclipse, often by observers stationed 

 within a few yards of each other, showed such discrepancies 

 as to suggest to some the theory that the corona is not 

 really a solar appendage, but an optical phenomenon, 

 caused by the passage of the sun's rays through our own 

 atmosphere ; and although a very slight knowledge of 

 mathematics sufficed when applied (for many mathema- 

 ticians, failing to apply their knowledge, were long misled) 

 to show the erroneous nature of this theory, it was not 

 until photography had been employed to delineate the 

 corona, that the groundlings were convinced on this com- 

 paratively simple point. A singular illustration of the in- 

 feriority of the unaided vision in this matter was given once 

 at a meeting of the Astronomical Society. " Two people," 

 said Mr. Stone, referring to the eclipse of 1875, which he 

 had witnessed in South Africa, " w*re asked to make 

 drawings of the corona, and at the end of the time one man 

 had drawn it in one shape, and the other as different 

 as it could possibly be." Considerable amusement was 

 caused by Mr. Stone's sketching two absurdly dissimilar 

 pictures on the blackboard. " The person who drew one 

 picture was an engineer, and was sitting side by side with 

 the other. Before the eclipse was over he turned round to 

 look at the other's drawing, and said, ' ^^'hat on earth are 

 you doing here ? ' He replied, ' I am drawing the outer 

 corona.' He said it was nothing of the kind ; yet he looked 

 and saw the outer corona just as the other had drawn it ; 

 but he had overlooked that it was the inner corona he was 

 drawing, until his attention was called to it in that way." 

 On tlie same occasion Mr. Stone pointed out another 

 defect of ordinary vision, as an instrument of scientific 

 research, which is well worth noticing. He said very truly, 

 that as soon as any feature has arrested the attention, it 

 becomes prominent ; another person may not catch the 

 same feature, and, passing it over, seizes upon some other 

 point and brings it out more strongly. Photography 

 has no weakness of this kind, at least where due care 

 is taken to use the same kind of plates and instruments, 

 nearly equal in size and similar in quality, when compari- 

 son is to be made between pictures taken by diU'erent 

 observers or at different stations. 



PHOTOGRAPHY FOR AMATEURS. 



By a. Brothers, F.R.AS. 



PART X.— TUE GELATINE PROCESS. 



rr^lHE photographic world is chiefly indebted to Mr. 

 J. Keunett for this process. In it wo liave one of the 

 most simple and perfect of any of the photographic pro- 

 cesses. The time of exposure in ordinary practice is 

 reduced to at least one-tentli of what was required for wet 

 collodion. This degree of rapidity may bo greatly ex- 

 ceeded, but the beginner will find the ten-times plates quite 

 rapid enough for ordinary work. 



"The first thing to attend to is to see that the room in 

 which the plates are to be manipulated is sufficiently dark. 

 There are many ways of effecting this. If the room liavp 

 ruby glass in the window, one or two thicknesses of yellow 

 paper as an extra covering may be sufficient. Daylight 

 may be altogether excluded and the work conducted by 



artificial light Lamps are sold for the purpose ; or one 

 may be made by cutting the neck and bottom from a hock 

 bottle, which only requires a suitable stand to admit air, 

 and a cover for the top, to stop the direct light of a candle 

 or small paraffin lamp placed within. There may be plenty 

 of light in the room, provided it be of the proper quality. 



It is important that the dark slide containing the sensi- 

 tive plate be covered with a black cloth when carried into 

 the daylight, as the smallest hole or crevice in the slide or 

 camera would admit sufficient light to fog the plate. The 

 plates may be developed by the ferrous-oxalate method, or 

 with pyrogallic acid, and a bromide and ammonia. 



Instead of holding the plate in the hand while developing, 

 as with wet collodion, a shallow disii is used, which need 

 be only just a little larger than the plate, in order to 

 economise the solutions. Three dishes will be necessar)* — 

 one to develope in, one to contain solution of alum, and the 

 other for fixing. 



Some operators prefer the ferrous-oxalate developor, 

 which is composed of oxalate of potash dissolved in warm 

 water to saturation, and in this is also dissolved oxalate of 

 iron (ferrous-oxalate) till the solution will take up no mora 

 This solution may be purchased ready for use, if preferred. _ 



The plate having been exposed, and everything necessary 

 got ready, it may be placed, face upwards, in the dish, 

 and sufficient of the developing solution may be poured into 

 the dish, so as to completely cover it After a few minutes, 

 the negative may be examined, and when sufficient density 

 appears to have been obtained, it may be removed from the 

 dish, and the oxalate solution returned to the stock-bottle. 

 The negative must now be rinsed with water, and then 

 put into the solution of alum, which is made by dissolving 

 alum in warm water until no more will dissolve. Of course, 

 all solutions must be used cuJd. 



Some gelatine plates have a tendency to frilL This is 

 caused by the expansion of the gelatine film by the absorp- 

 tion of moisture. The effect of the alum solution is to 

 harden the film. If the plate be touched with the finger 

 while wet with the developing solution, it will be smooth 

 to the touch ; but after immersion in the alum for a minute 

 or two, when touched again, it will be noticed that the 

 character of the surface has changed, and this indicates 

 that the film is sufficiently hardened. The plate may now 

 be rinsed under the water-tap, and is ready for the next 

 operation — fixing — and this is effected l>y immersion in a 

 saturated solution of hyposulphite of soda (or thiosulphnte 

 of soda, to use the correct name). 



This method of development has the advantage of being 

 clean and very easy to manage, but it has the disadvantage 

 that the solution is not constant, and the results are not 

 always to be relied on. Of course, it is possible to become 

 so familiar with a process that difficulties in the hands of 

 one operator are not met with by another. 



Development with pyrogallic acid is the method in 

 most general use. Jlr. Kennett's formula is : — Solution 

 No. 1, pyrogallic acid 4 grains, water 1 oz. Solution L', 

 ammonia (sp. gr. -t^SO) half an oz., water 8 oz. Solution 

 .'?, potassium bromide 3 drs,, water 8 cz. Solutions 2 and 

 3 may be kept mixed together. It is not necessary to 

 weigh the pyrogallic acid for every plate. Keep a small 

 bone spoon for the purpose, and after once weighing the 

 proper quantity, notice wliat is required for the si/e of jilate, 

 and with the spoon sufficient may be yitessnf, a small 

 quantity more or less being of very little consequence. 

 But if Mr. Kennett's method bo followed, lake of No. 1 

 one ounce, and add to it one dram (or drachm) of tlie 

 amuionia and bromide solution, which must be poured over 

 the plate while lying flat in the tray. If any air-bubbles 

 appear on the plate, they must be carefully removed, and the 



