IN THE CAMERA.] TJNDULATORY FORCES. LIGHT PHOTOGRAPHY. 



149 



Many writers have recommended a less proportion of 

 nitrate of silver ; others have advised the addition of 

 more iodide of potassium : some recommend the use of 

 a little acetic acid and alcohol to such a solution. We can 

 only state, that our own opinion is, that all these additions 

 are, and have been, recommended to meet cases which 

 may, or may not, occur to other experimenters. For 

 instance, the nature of the collodion incessantly varies, 

 by reason of the chemical changes to which it is subject ; 

 and with every desire to assist the beginner, we must 

 state, that experience, patience, and watchfulness, are 

 the sole guides or assistants on which he can at all 

 depend. We shall, however, give a variety of recipes 

 for these and other solutions which have been proposed 

 by various experimenters, in another part of the work, 

 leaving to the student the chance of trying each until an 

 average of success attends his labours. 



The solution having been prepared, it should be 

 rig. 156. poured into an upright bath or 



trough, such as is illustrated 

 in the annexed engraving. 

 These are generally made of 

 gutta-percha ; but an upright 

 glass, or porcelain trough, will 

 answer just as well. The 

 length and breadth, of course, 

 must depend on the size of the 

 glass. If the vessel be up- 

 right, and not oblique, it should 

 not be less than half an inch 

 wide. Sufficient silver solu- 

 tion should be employed as will nearly fill the vessel ; 

 and, of course, it should be in such a quantity as will 

 cover the whole of the glass when introduced therein. 

 Some operators use flat dishes for this purpose; but 

 they run risks of spoiling the plate, owing to dust, <tc. , 

 settling on the surface of the solution. 



When an upright trough is employed, the plate should 

 be placed in a frame, for the purpose of supporting it 

 whilst remaining in the solution. This arrangement 

 prevents the necessity of touching the plate, or of 

 wetting the finger ; and, as such, is conducive to cleanli- 

 ness and convenience. One of these frames is repre- 

 sented on the outside of the trough in Fig. 156. 



The plate having been coated with collodion, should, 

 whilst still wet, be placed in the frame, and must then 

 be deadily and continuously introduced into the silver 

 bath. This should not be hastily done, lest the collodion 

 film may be removed or injured ; and no stoppage should 

 be allowed, because that would produce a distinct and 

 damaging mark on the surface. Of course this must be 

 done in the dark room. After leaving the plate in the 

 Kith for about a minute, it should be gradually with- 

 drawn ; and after remaining a few seconds in the air, is 

 again to be introduced, and may be left there for about 

 two minutes longer. On removal, it should present an 

 even opal appearance, and be perfectly free from spots, 

 lines, <to., or otherwise it will be quite useless. The 

 length of time during which the plate should be in the 

 bath, depends on a variety of circumstances ; and hence 

 we have only mentioned an average, leaving to the ex- 

 perience of the operator to decide in special instances. 

 It should then be placed in the dark frame, the lid of 

 which should be carefully closed ; and no time must be 

 lost in transferring it to the camera, lest its sensitiveness 

 may become lessened. 



4. Exposing the Plate in the Camera. The plate 

 having been prepared, and placed in the dark frame, 

 is now ready to be transferred to the camera; 

 and in removing it, a cover of black cloth or velvet 

 should be thrown over the whole before removal 

 from the d.-irk room. This precaution prevents any 

 chance of a ray of light entering through any imper- 

 fection in the construction of the frame itself. The 

 operator shouM again examine the image formed in the 

 camera; for although only two or three minutes may 

 have elapsed since he focussed his lenses, still persons 

 are very apt to shift from their position ; and therefore, 

 of course, by so doing they will lessen the sharp ap- 



pearance of their image on the ground glass. The 

 sitter must be requested to remain perfectly still ; 

 and having arranged him in the most advantageous 

 position, and refocussed the lenses, the brass cap in 

 front of xhese should be put on to the tube, so as to 

 prevent the ingress of any light into the body of the 

 camera. The ground-glass frame is now to be removed, 

 and the prepared plate must take its place. It is best 

 put into position by lifting it up with the black cloth, 

 which should be dextero\isly covered over the top of the 

 camera, whilst the frame containing the plate is inserted 

 in the slide. The lid or flap in front of the plate is now 

 to be removed ; and after again enjoining complete 

 stillness on all present, gradually remove the brass 

 cap in front of the lenses. The rays of light will now 

 fall on the sensitive plate, and produce their chemical 

 effect. 



The student will expect that we should here give him 

 minute directions as to the length of time during which 

 he must expose the plate ; but this it is impossible for 

 us to do. The weather, both with respect to the amount 

 and colour of daylight, has a great effect : then, again, 

 the nature of the collodion employed, the colour of the 

 face and dress of the sitter, and various other contin- 

 gencies, render the question of great uncertainty. The 

 same collodion used on two different days, or at different 

 periods in the same day, may produce different results. 

 As a general rule for the beginner, an exposure of from 

 fifteen to thirty seconds may be adopted ; and as he gains 

 experience, he will soon be enabled to estimate the value 

 of the different causes which tend to render a picture 

 more or less perfect. We shall have to speak more fully 

 of this presently, when we refer to the developing pro- 

 cess. Presuming that the plate has been sufficiently ex- 

 posed, the brass cap should now be replaced in front of 

 the lenses ; the flap is to be shut down in front of the 

 plate ; and the frame should be removed from the camera, 

 so that the black cloth shall completely cover it The 

 frame, etc., are now to be removed into the dark room, 

 for the purpose of development. 



5. Developing the Picture. The tyro will very 

 likely imagine, that on removing the plate from the 

 frame, he will observe the picture at once on the 

 prepared surface. Such, however, never occurs: the 

 plate presents, as near as possible, the same appear- 

 ance after, as before, its exposure to light; ant! the 

 latent picture must, therefore, be developed that is, 

 brought out into a sensible form by means of further 

 chemical action. The philosophy of this process is easy 

 of explanation : the iodide of silver, formed on the 

 surface of the collodion when it was immersed in the 

 silver bath,* has now become changed. That portion 

 which has received the rays of light, will be differently 

 acted on, by the addition of certain agents, to that ou 

 which no change has taken place by luminous rays. On 

 pouring the developing solution on to the surface, this 

 difference gradually becomes apparent as the picture 

 unfolds itself on the glass surface. But we now proceed 

 to the practical part of the operation. 



The door of the dark room being closed, and 

 only a faint yellow light em- 

 ployed, the lid of the frame 

 should be removed, and the 

 glass plate is to be carefully 

 taken out, and placed, with 

 the uncoated surface down- 

 wards, on a level stand, rest- 

 ing in a large porcelain or 

 glass dish. Such an arran^" 

 ment as this is represented in ^ 

 the annexed engraving. 



In the absence of a stand of this sort, two glass rods 

 may be placed across the edges of the vessel ; and on 

 these the plate may rest ; or even an inverted tumbler or 

 wine-glass may be employed. The object is to keep the 

 plate quite horizontal during the development, and to 

 receive, in the dish beneath it, the superfluous developing 

 fluid which will run from the plate. 



Sen anti, p. 148. 



Fin. 1*7. 



