SEirsrrisnro THE PLATE, ETC. j UNDULATORY FORCES. LIGHT PHOTOGRAPHY. 



157 



into the glass tray at the bottom of one of the boxes ; 

 and having fitted the silver plate, face outwards, in the 

 frame, place it horizontally over the tray containing 

 the iodine. The vapour which arises will reach the 

 silver surface, forming an iodide of silver, which should 

 be of a rose colour. The time required to effect this 

 varies according to temperature and other circum- 

 stances ; and so, like many other photographic pro- 

 cesses, must be left to the judgment of the operator. 

 As soon as the rose tint is obtained, the plate may 

 then have its sensitiveness increased by means of the 

 vapour of bromine, <fec. ; and such have been termed 

 accelerators, from their power of rendering the sub- 

 sequent processes more rapid of execution. After being 

 iodised, the plate becomes sensitive to the action of 

 light, but not to any great extent. It is, however, 

 desirable that iodising should be carried on in the dark 

 room ; whilst it is essential that the accelerating process 

 should be so conducted. Great care should be taken 

 that as little as possible of the vapour of iodine and 

 bromine should escape into the room, because such 

 not only may have a generally prejudicial action on the 

 plates, but also on various metals, chemicals, <kc. ; 

 besides the personal inconvenience and injury which 

 would occur to the operator. 



The accelerating effect is produced by exposing the 

 plate, previously iodised, in another box, which has a 

 glass tray, into which the accelerating solution is to be 

 poured. The exposure is conducted in precisely the 

 same manner as in the process of iodising, and a rich, 

 hut fuller, dark rose-red surface is obtained on the silver 

 plate, which will then be nearly ready for exposure in 

 the camera. 



Arcderating Sctvtimu. A solution of bromine in 

 water may be employed for this purpose ; and it is made 

 by adding some bromine to distilled water, and shaking 

 the two together until as much bromine as possible has 

 been taken up. One pint of this saturated solution is 

 to be diluted with distilled water, until it presents a 

 pale straw colour ; and then poured, to the depth of half 

 an inch, in the glass tray. 



Another solution may be prepared by adding a few 

 (1 n >ps of hydrochloride acid to a drachm of bromine, which 

 is then to be diluted with six ounces of distilled water. 



By using the above solutions as accelerators, a rich 

 red rose-colour is produced on the plate. The latter 

 must afterwards be exposed again to the iodine vapour, 

 until the silver acquires, a very deep roue-red, or, occa- 

 sionally, a blue colour, resembling that produced on 

 steel when it has been heated to a temperature of 

 between 600 and 700 degrees Fah. ; and the iodising 

 process is then finished. 



An accelerator recommended by M. Claudet, has the 

 advantage of preventing the necessity of a second ex- 

 posure to the iodine vapour. A portion of it is poured 

 into the glass tray of the accelerating box, and after 

 the silver has been exposed to the action of the vapour 

 for several minutes (even to the extent of half-an-hour 

 occasionally), it is ready for the camera. The mixture 

 consists of a saturated solution of iodine in bromine ; 

 which is easily done by adding as much iodine to the 

 liquid bromine as it will take up. A bromide of iodine 

 is thus formed, and as much of it is added to distilled 

 water as will impart to the latter a full yellow colour. 

 It is stated, that this plan is always to be depended 

 upon ; and it has one advantage for the tyro which is, 

 that he can scarcely err in its employment. 



We must here point out a few matters by way of cau- 

 tion. The iodine should be spread evenly on the glass tray; 

 because, wherever it is in masses, it will produce a 

 deeper tint over that part. Care must be taken not 

 to disturb the particles of iodine by hastily placing the 

 plate on the top of the box, as this might possibly 

 cause the air inside to be disturbed, and so drive par- 

 ticles on to the surface of the plate, where it would 

 produce spots, and entirely destroy it* fitness for sub- 

 sequent processes. 



3. Exporure in the Camera. A sensitive plate having 

 been produced, the next step u its exposure in the camera. 



On this head we have already made several remarks 

 whilst directing the reader in conducting the collodion 

 process. The same precautions and arrangements are 

 required. The silver plate, however, is not so sensitive 

 to the action of light as those of highly sensitised 

 collodion. The time of exposure varies according to a 

 variety of circumstances, each of which, as it presents 

 itself, becomes the subject of annoyance to the operator, 

 but yet adds to his knowledge and experience. We 

 shall, therefore, not further deal with this part of the 

 whole process, but refer our readers to the remarks we 

 have made at a previous page.* We may, how- 

 ever, remark, that great care is required to prevent 

 the effect called solarisation, which is due to over-ex- 

 posure. This entirely destroys the contrast of light 

 and shade, on which depends the beauty of all pho*x> 

 graphed productions. However, to prevent this, the 

 operator must be well acquainted with the power of 

 his lens, and the excellency of the plates and che- 

 micals ; matters which, of course, can only be gained 

 by practice and experience. 



4. The Mercury Process. On removing the plate from 

 the camera into the dark room, no picture will be 

 observed on its surface ; it must therefore be exposed 

 to the vapour of mercury, which, settling on the face of 

 the plate, rests on those parts which have been affected 

 by the action of light. For this purpose a peculiarly 

 shaped box is employed. In the lower part of it is a 

 cistern, containing about two ounces of quicksilver ; and 

 in this the bulb of a thermometer is placed, which indi- 

 cates the temperature of the mercury when heated by 

 means of a spirit-lamp placed beneath it. This tem- 

 perature should never be allowed to rise beyond 150 or 

 160 Fah. The plate is placed at an angle of 45 at 

 the top of the box. 



Presuming that a plate has been duly exposed to 

 light, and is allowed to receive the vapour of mercury 

 in the manner we have explained, we shall soon perceive 

 that the parts which have been chiefly acted on by 

 light, are most deeply brought out by the mercurial 

 vapour, and every other part receives its due propor- 

 tion, and produces a corresponding effect. 



By these means the latent picture becomes developed ; 

 the effects of light and shade are produced on the surface 

 of the plate, and a representation of the object copied 

 is afforded, correct and brilliant, just in proportion to 

 the care which has been exercised by the operator in 

 each process, and in the actinic power of the light 

 which has produced the picture. 



6. fixing the Picture. The next part of the process is 

 that of fixing the picture, and of so preventing the 

 further action of light on the plate. The only thing 

 required is to remove the remainder of the sensitive 

 silver salt from the face of the plate ; and this is 

 effected by immersing the plate, when cool, in a solu- 

 tion of one part of hyposulphite of soda, to from six to 

 ten parts of water. It should be left in the solution 

 for a few minutes, and then must be abundantly 

 washed with cold water. This is best effected by allow- 

 ing a gentle stream, from a water-tap, to impinge on the 

 plate : by these means all the soluble salts are com- 

 pletely removed. As, however, common water contains 

 lime, ifec. , the plate should be finally washed with dis- 

 tilled water ; and it may then be dried by holding it at 

 some distance over the flame of a spirit-lamp. In dry- 

 ing, great care should be taken that no drops of water 

 are left on the surface, as they would be sure to form 

 spots. As the plate heats, the water has a tendency to 

 run off from its surface ; and an attentive operator will 

 take advantage of this fact, so as to prevent the occur- 

 rence of spots, <bc. , thereon. 



The appearance of all Daguerreotypes is remarkably 

 improved by submitting them to the action of chlo- 

 ride of gold, after they are removed from the hypo bath. 

 The contrast of light and shade is thereby greatly en- 

 hanced, and the brilliancy of the picture is of course 

 increased. The process is very simple, and may be per- 

 formed as follows : 



See antt, p. 148. 



