l.-.l 



UNDULATORY FORCES. LIOHT PHOTOGRAPHY. [THE FIXING FROCKB. 



place, it will remain without change for ;iuy length 

 of time. We need scarcely say that it is not liable to 

 alter l>y the action of light. 



The sensitising process, or that by means of which 

 the salted paper is rendered fit for the action of light, 

 should bo performed shortly before tin- paper is required 

 for use, because animal matters, as wo have already 

 remarked.* have a spontaneous action on salts of silver. 

 The solution may be as follows, viz. 



te of silver 80 to 100 grains. 



Distilled water 1 fluid ounce. 



This is to be poured into a clean glass or porcelain 

 Teasel ; and a piece of the paper, prepared as we have 

 described, is to be placed on its surface in the same 

 manner as that recommended for its allximcnisation, 

 care being taken that m> air-bubbles are left on the 

 surface, and that no part be touched by the finger, 

 except the turued-up corner. iThe paper may bo left 

 in tho solution for a few minutes, and is then to be 

 dried in a dark place, because, owing to tho formation 

 of the chloride of silver on the surface, it lias become sen- 

 sitive to light Of course, this part of the process should, 

 for such reasons, be entirely carried on by candlelight. 



The strength of tho silver solution gradually di- 

 minishes as each sheet is taken from it, and therefore 

 an occasional addition of some more nitrate of silver, 

 or of a strong solution of that salt, will be required. 

 When the solution becomes of a brown tint, its colour 

 may be restored by adding to it a small portion of pipe- 

 clay. This is to be shaken up with the liquid, which 

 can afterwards be filtered through some blotting-paper, 

 if requisite. 



3. The Copying Proctss. To effect this successfully, 

 a copying frame should be employed. It consists of an 

 outer frame, which holds a sheet of plate-glass. The 

 negative is placed on the glass with tho collodion side 

 next to the paper. The prepared paper is placed with 

 its albumeuised side next to the negative ; and to keep 

 the two in close contact, a back fits into the frame, and 

 is secured by means of two cross pieces therein. Folds 

 of blotting-paper, or soft cloth, may be placed between 

 the paper and back of the frame, so as to press the 

 paper firmly against tho negative plate. This is of 

 great consequence, because any crease in the paper 

 would at once spoil the print, and produce general in- 

 distinctness of outline. The back of the frame is made 

 to open and shut ; so that whilst one part still firmly 

 presses on the plate and paper, another part may be 

 raised, and the progress of the printing examined. 



We can give no definite directions as to the length 

 of time required for exposure. We have already often 

 remarked on this point. To illustrate this, we may 

 instance the numerous experiments which have been 

 made for the purpose of testing the question. In the 

 morning of a summer day, in clear sunshine, the printing 

 process has been effected in a few minutes ; whilst after 

 four o'clock, it has required an exposure of from half 

 to three-quarters of an hour to produce the same effect. 

 Occasionally we have noticed an almost entire suspen- 

 sion of actinic effect; but, a few moments afterwards, 

 whilst the sun has been obscured by a thin cloud, the 

 printing seemed to have gone on with increased vigour. 



In the hope of giving something like an idea on this 

 point, we have had collodion positives taken during 

 v quarter of an hour, from ten in the forenoon, 

 with little intermission, till six in the evening, in the 

 early part of Juno, choosing alternately a clear sun- 

 shine and dark cloudy weather ; and despite all attempts 

 at uniformity in every circumstance, so far as manipu- 

 lation and chemicals were concerned, our results entirely 

 baffled any hope of affording constant rules. 



Generally speaking, it is better that the positive 

 should be over, rather than under-printed, because, when 

 placed in the fixing solution, it undergoes consi<l. 

 modification, and every part becomes of a lighter tint. 

 Of course, much depends on tho depth of shade, <tc., in 

 the negative, i ither circumstances being equal. We must 

 therefore leave this point to the care and judgment of the 

 See unit, p. 140. 



i. who will, l>y practice, arriveat soni. cer- 



>f result, and which experience alone can give. 

 During the printing process, the paper may be ex- 

 amined in the way we have mentioned, or a piece of 

 paper as that in tho frame, may bo exposed 

 under .-in equally thick piece of plate-glass, and tho 

 process of colouring be watched ; unless so placed it will 

 deceive the operator, because tho glass of the framn 

 exercises a retarding effect on tho progress of the 

 chemical changes. 



A usual form of copying frame is illustrated in the 

 following engraving : 



Fig. 159. 



a 



a if the outside frame ; & i the back of the frame ; e e, two bars, which 

 fit into the sides of the frame, and BO i < 



j:>-ther ; ti is a hinge, by means of which, cither end of the back may be 

 up' nod, so as to allow of the positive being examined during the pro- 

 gress of printing. 



4. The Fixing Process. Presuming that tho positive 

 has been properly printed, tho next step is to prevent 

 further action of light thereon, and so to render the 

 picture permanent. This is effected by immersing it 

 in a bath of hyposulphite of soda, which dissolves 

 away all tho unchanged chloride of silver. The pro- 

 portions of the solution are as follow : 



Hyposulphite of soda . . .1 ounce. 

 Distilled water .... 4 ounces. 

 On removing the print from the copying frame, it 

 should be abundantly washed with cold water, which 

 removes all substances soluble in that liquid. A solu- 

 tion of half an ounce of common salt, and a small 

 quantity of bicarbonate of soda, is then made in a pint 

 of water, into which the print is to be dipped, to free 

 it from all traces of nitrate of silver and nitric acid. 

 It should then be again washed in abundance of water, 

 to free it from the soluble salts it may contain. The 

 next step is to immerse it, face downwards, in the hypo- 

 sulphite bath, in which it may remain for a quarter of 

 an hour. It, however, should be continually examined, 

 because tho fixing solution has a tendency to destroy 

 the distinctness of outline and gradation of shadow 

 which gives the beauty to a good print. During this 

 process, stains occasionally appear, which are owing to 

 the accidental presence of free nitrate of silver. This 

 should have been prevented by the salt and soda bath ; 

 or they may have been caused by carelessness in touch- 

 ing the print with the fingers. As hyposulphite of 

 soda is a cheap material, it is better to use a fresh solu- 

 tion for every other picture. This plan often prevents the 

 production of stains, and generally ensures good results. 

 It only remains that the print should be afterwards 

 well cleansed from all hyposulphite of soda, which is 

 done by placing it on tho surface of clean water, and 

 pouring a gentle stream on it for some time. The print 

 i.-i i hen left for an entire day in a vessel containing 

 i V of water, which should be occasionally changed. 

 This having been effectually performed, the picture 

 may bo considered as permanently fixed. 



6. The Toning Proctss. We have as yet described 

 what maybe termed the ordinary process of printing: 

 there, however, remain for us to mention various plans, 

 by means of which tho brilliancy of tho positive may be 

 ui.-reasod. This is usually effected by means of a solu- 

 tion of chloride of gold, in tho manner which wo proceed 

 to describe. Wo must specially observe, that groat 

 care is required to produce tho boat results ; and tho 

 tyro must not expect, at a first trial, to arrive at per- 

 oii. For this reason we have deferred introducing 

 the subject of toning until we supposed, by the practice 

 of previous processes, some amount of proficiency had 

 been acquired. 



