CHAMBERS'S INFORMATION FOR THE PEOPLE. 



process is the development of the, as yet, invisible 

 image. For this purpose, the plate is taken into 

 the dark room, and held by the corner, as when 

 about to be coated with collodion, except that 3 

 and 4 are held a little lower than I and 2. The 

 developing solution may consist of iron proto- 

 sulphate, 5 drachms ; glacial acetic acid, i ounce ; 

 alcohol, i ounce; and water, 18 ounces. A little of 

 this is put into a small tumbler or developing-glass, 

 and poured with a rapid sweep over the plate, 

 the glass being moved from 2 to i, so as to prevent 

 the solution falling on one spot, and the plate 

 at the same instant being made level, to keep it 

 from running off at 3 and 4. A gentle rocking 

 motion should now be given to the plate, to keep 

 the solution moving ; and if the exposure has been 

 sufficient, the image will appear in from 5 to 10 

 seconds, feeble at first, but gradually getting 

 stronger, till sufficient density is obtained. The 

 plate is then well washed under a tap, and the 

 image fixed by immersion in a dish of, or by pour- 

 ing on, a solution of potassium cyanide, 1 5 grains ; 

 water, I ounce ; or a saturated solution of soda 

 hyposulphite. The fixing is complete as soon as 

 the yellow iodide of silver is dissolved ; and the 

 plate is again washed thoroughly in plenty of 

 water, and set up to dry, and then only requires 

 varnishing to be ready for printing. 



Occasionally, it occurs that sufficient printing 

 density cannot be obtained by the application of 

 the developing solution. In that case, the iron 

 should be poured off, and the plate well washed, 

 and the following intensifying solution applied : 

 pyrogallic acid, 3 grains ; citric acid, 2 grains ; 

 water, i ounce. To 2 drams of this, immediately 

 before pouring it on, add 5 drops of a 3O-grain 

 solution of nitrate of silver, and keep it moving on 

 the plate till sufficient density is obtained. 



POSITIVE COLLODION PROCESS. 



The aim of this process, which, in its manipu- 

 latory details, differs in no respect from that for 

 negatives, just described, is the production of 

 positive or direct pictures on glass. It is simple, 

 and the results, when skilfully obtained, are very 

 beautiful. Especial care must be taken to have 

 the plates thoroughly clean. The collodion used 

 for negatives will do if diluted with 20 per cent, 

 of a mixture of ether and alcohol, and is improved 

 by the addition of sufficient free iodine to give it 

 the colour of pale sherry. The exposure for a 

 positive is from a half to a third less than for a 

 negative ; and although the negative developer, 

 with two drops of nitric added to each ounce, 

 gives good results, the following is better : Proto- 

 sulphate of iron, 7 drachms ; distilled water, 

 jo ounces ; nitrate of baryta, ounce ; nitric 

 acid, i ounce; alcohol, I ounce. Dissolve the 

 nitrate of baryta in the water, add the nitric acid, 

 and then the protosulphate and spirit, and filter 

 from the white precipitate. Pour one or other of 

 these solutions on the plate as evenly as possible, 

 keeping it in motion. Observe when the lower 

 lights appear, and stop the action immediately by 

 copious washing with water. Fix with water, 10 

 ounces ; cyanide of potassium, I drachm. This 

 solution can be used over and over again, by 

 adding occasionally a little fresh cyanide, to keep 

 up the strength. Wash the plate freely, and set it 



782 



aside to dry; after which, protect the picture 

 with varnish, and cover the back of the plate with 

 the common jet-varnish used for protecting iron- 

 work, &c. ; or place behind it a piece of black 

 velvet. The picture is lastly to be mounted in 

 some sort of frame. If well washed after fixing, it 

 should be quite permanent. 



Positive Printing Processes. 



These have as their object the obtaining from 

 a negative on paper, glass, or other transparent 

 media, a transfer or direct picture, as formerly 

 defined. This is done by superimposing the 

 negative, with the picture-side downward, upon 

 any surface sensitive to light, and in as close 

 contact with it as is consistent with the safety of 



Fig. 14. 



the negative, and exposing them to light for a 

 length of time varying with the nature of that 

 surface. In printing upon dry paper, a printing- 

 frame (fig. 14) is used. A is a strong frame of 

 wood, of any convenient size, in the bottom of 

 which is fixed a sheet of thick plate-glass. B is a 

 board which fits loosely into the frame, and is 

 generally covered on the under side with thick 

 velvet or felt. It is in two pieces, joined together 

 by hinges, so that one half may be opened for the 

 purpose of examining the progress of printing, 

 without risk of shifting the paper. C, C, are two 

 bars, hinged to the frame at one end, fastened at 

 the other by hooks. Each bar is furnished with a 

 spring, the pressure of which keeps the prepared 

 paper and negative in close contact. 



Positive prints are now almost exclusively made 

 on albuminised paper, and therefore we shall con- 

 fine ourselves to a description of its manufacture 

 and manipulation. It is of the utmost importance 

 to get a suitable sample of paper. At the pres- 

 ent time there are only two kinds in general use, 

 Papier le Rive and Papier Saxe, both of con- 

 tinental manufacture, and both very good. Take 

 egg-albumen, 18 ounces; water, 2 ounces; ammo- 

 nium chloride, i oo grains. Dissolve the salt in the 

 water, add it to the albumen, whisk the whole into 

 froth in the ordinary way, and when it has sub- 

 sided, decant the clear part into a flat dish. Take, 

 now, a sheet of paper, holding it by two opposite 

 corners, and place the middle of it in contact with 

 the albumen ; then let the corners drop ; raise the 

 sheet partially, and remove air-bubbles, should 

 any have been formed, by means of a camel- 

 hair brush. Allow the sheet to remain upon 

 the albumen for two minutes, and pin it up by 

 a corner to dry spontaneously. This albumin- 

 ised paper will keep good for a very long time, if 

 preserved clean and dry in a portfolio. To render 

 sensitive, a sheet is placed in the same manner 

 as in albuminising, upon the following solution : 

 water, i ounce ; nitrate of silver, 60 grains air- 

 bubbles being carefully avoided, as before. After 



