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en exposed to light； and we are thus provided with a medium by which； with the aid of the photogenic camera； we may effectually copy views from nature； figures； buildings； and even take portraits from the shadows thrown on the paper by the living face。 The paper may be used somewhat damp。 The best plan for fixing it in the camera is to place it between a piece of plate glass and some other material with a flat surface； as a piece of smooth slate or an iron plate； which latter； if made warm； renders the paper more sensitive； and consequently the picture is obtained more rapidly。
Time of Exposure。With regard to the time which should be allowed for the paper to remain in the camera； no direct rules can be laid down； this will depend altogether upon the nature of the object to be copied； and the light which prevails。 All that can be said is； that the time necessary for forming a good picture varies from thirty seconds to five minutes； and it will be naturally the first object of the operator to gain by experience this important knowledge。
Bringing Out the Picture。The paper when taken from the camera； which should be done so as to exclude every ray of lightand here the dark slide of the camera plate holder becomes of great use bears no resemblance to the picture which in reality is formed。 The impression is latent and invisible； and its existence would not be suspected by any one not acquainted with the process by previous experiment。 The method of bringing out the image is very simple。 It consists in washing the paper with the gallo…nitrate of silver； prepared in the way already described； and then warming it gently； being careful at the same time not to let any portion become perfectly dry。 In a few seconds the part of the paper upon which the light has acted will begin to darken； and finally grow entirely black； while the other parts retain their original color。 Even a weak impression may be brought out by again washing the paper in the gallo…nitrate； and once more gently warming it。 When the paper is quite black； as is generally the case； it is a highly curious and beautiful phenomenon to witness the commencement of the picture； first tracing out the stronger outlines； and then gradually filling up all the numerous and complicated details。 The artist should watch the picture as it developes itself； and when in his judgment it has attained the greatest degree of strength and clearness； he shall stop further proceedings by washing it with the fixing liquid。 Here again the mixed solution need not be used； but the picture simply brushed over with the gallic acid。
The Fixing Process。In order to fix the picture thus obtained； first dip it into water； then partly dry it with bibulous paper； and wash it with a solution of bromide of potassiumcontaining one hundred grains of that salt dissolved in eight or ten ounces of distilled water。 The picture is again washed with distilled water； and then finally dried。 Instead of bromide of potassium； a solution of hyposulphite of soda； as before directed； may be used with equal advantage。
The original calotype picture； like the photographic one described in the last chapter； is negative； that is to say； it has its lights and shades reversed； giving the whole an appearance not conformable to nature。 But it is easy from this picture to obtain another which shall be conformable to nature； viz。； in which the lights shall be represented by lights； and the shades by shades。 It is only necessary to take a sheet of photographic paper （the bromide paper is the best）； and place it in contact with a calotype picture previously rendered transparent by wax or oil as before directed。 Fix it in the frame； Fig。 29； expose it in the sunshine for a short time； and an image or copy will be formed on the photogenic paper。 The calotype paper itself may be used to take the second； or positive； picture； but this Mr。 Talbot does not recommend； for although it takes a much longer time to take a copy on the photogenic paper； yet the tints of such copy are generally more harmonious and agreeable。 After a calotype picture has furnished a number of copies it sometimes grows faint； and the subsequent copies are inferior。 This may be prevented by means of a process which revives the strength of the calotype pictures。 In order to do this； it is only nesessary to wash them by candlelight with gallo…nitrate of silver； and then warm them。 This causes all the shades of the picture to darken considerably； while the white parts are unaffected。 After this the picture is of course to be fixed a second time。 It will then yield a second series of copies； and； in this way； a great number may frequently be made。
The calotype pictures when prepared as we have stated； possess a yellowish tint； which impedes the process of taking copies from them。 In order to remedy this defect； Mr。 Talbot has devised the following method。 The calotype picture is plunged into a solution consisting of hyposulphite of soda dissolved in about ten times its weight of water； and heated nearly to the boiling point。 The picture should remain in about ten minutes； it must then be removed； washed and dried。 By this process the picture is rendered more transparent； and its lights become whiter。 It is also rendered exceedingly permanent。 After this process the picture may be waxed； and thus its transparency increased。 This process is applicable to all photographic papers prepared with solutions of silver。
Having thus fully； and it is hoped clearly； considered the process； it may be necessary before dismissing the calotype from notice； to add one or two remarks from the observations and labors of some who have experimented in this art。 Dr。 Ryan in his lectures before the Royal Polytechnic Institution； has observed； that in the iodizing process the sensitiveness of the paper is materially injured by keeping it too long in the solution of iodide of potassium； owing to the newly formed iodide of silver being so exceedingly solvable in excess of iodide of potassium as in a few minutes to be completely removed。 The paper should he dipped in the solution and instantly removed。 There is another point； too； in the preparation of the iodized paper in which suggestions for a slight deviation from Mr。 Talbot's plan have been made。 In the first instance； it is recommended that the paper be brushed over with the iodide of potassium； instead of the nitrate of silver； transposing； in fact； the application of the first two solutions。 The paper； having been brushed over with the iodide of potassium in solution； is washed in distilled water and dried。 It is then brushed over with nitrate of silver； and after drying is dipped for； a moment in a fresh solution of iodide of potassium of only one…fourth the strength of the first； that is to say； one hundred and twenty…five grains of the salt to a pint of water。 After this it is again washed and dried。 The advantage derived from this method； is a more sensitive paper； and a more even distribution of the compounds over the surface。
Another deviation from Mr。 Talbot's method has been suggested； as follows：
Brush the paper over with a solution of one hundred grains of nitrate of silver to an ounce of water。 When