TUB OALOTYPE. ] 



UNDULATORY FORCES. LIGHT PHOTOGRAPHY. 



159 



afterwards to be dried in a dark place, and, if not imme- 

 diately used (which is advisable), it should be kept out of 

 the rays of the sun, or of diffused daylight. As, how- 

 ever, organic matters tend to decompose silver salts, 

 even in the dark,* it is better that this paper should 

 only be prepared in such quantities as may be consistent 

 with the immediate requirements of the operator. 



The paper is then to be placed, with its silver side 

 down wards, on a solution of iodide of potassium (hydrio- 

 date of potass), in the proportion of 



Iodide of potassium . . 100 grains. 

 Distilled water ... 3 ounces. 



By these means the iodide of silver is formed on its 

 surface, which gives the paper a kind of primrose ap- 

 pearance. It is then dried, first being pressed between 

 folds of clean white blotting-paper. In this state 

 it is barely sensitive to the action of light ; but 

 the process should be conducted in a dark place, 

 such as by candlelight, or in a dark room. This we 

 term iodised paper ; and, if properly prepared, 

 and all the precautions with respect to cleanliness, <tc. , 

 having been observed, it may be kept for some time 

 without danger of undergoing any very serious change. 

 However, in our repetition of Mr. Talbot's experiments, 

 wo have found that the almost certain presence of organic 

 <;r in the size of almost every kind of paper, has a 

 tendency to produce changes which eventually exhibit 

 their efficts in spots, <fcc., on the surface of the pro; 

 paper, if such be made for any length of time before being 

 used. 



The reader will thus observe, that the calotype pro- 

 cess di -:>end.s on a formation of an iodide, in place of the 

 clilorM.j of silver, on the paper surface; and on this de- 

 pend some of the peculiarities of Mr. Talbot's calotype 

 process. 



Before exposure in the camera, this iodised paper must 

 bo rendered more highly sensitive ; and this is done by 

 placing tlie pr. -pared surface on a solution of 



Nitrate of silver . . . 100 grains. 

 Distilled water ... 2 ounces. 

 Acetic acid .... 3 drachma. 

 To which is to be added an equal quantity of a saturated 

 solution of gallic acid. We may here observe, that gallic 

 arid is but sparingly soluble in water. Mr. T 

 the mixture the gallo-nitrato of silver. This solution 

 should, whilst prepared in the proportions we have 

 named, be only made in quantities required for imme- 

 diate use, because it spoils by keeping. 



After the iodised surface has been rendered sensitive 

 by these means, the paper is to be dri. ! ; and this process 

 should be conducted shortly before the exposure of the 

 paper in the camera. It may be used with advantage 

 in a slightly moist state, which somewhat facilitates 

 the subsequent process. 



The paper may then be used in the camera, in the 

 manner already pointed out for the collodion and 

 lorreotype processes ; and the time of exposure will 

 vary according to the sensitiveness of the surface, and 

 the intensity of the light reflected from the object 

 about to be copied. A latent ima_;o will thereby be 

 formed ; and this is to be rendered visible by placing 

 the pa[r on, or by pouring over its surface, a solution 

 of the/ nitrate of silver, acetic, and gallic acids just 

 named. The photograph or calotype is thereby de- 

 veloped ; and this should bo done in a gradual manner. 

 Sometimes tin; development takes place somewhat irre- 

 gularly, and spots and stains appear. If the exposure 

 has not been sufficiently long in the camera, the picture 

 takes a considerable time Ixtfom it appears ; and, in 

 easoa, the aid of a gentle heat may be called in. The 

 picture may then be fixed by immersing it in ' a 

 solution of 



ide of potassium . . . 100 grains. 

 I water .... 10 ounces. 

 After which it is to be abundantly washed with water, 

 and subsequently dried. I!y these means a negative is 

 produced ; and by dipping the prepared paper in a solu- 

 tion of iodide of potassium, a positive may be produced, 



See ante, p. 140. 



providing the gallo-nitrate immersion has been adopted. 

 After exposure in the camera, the gallo-nitrate solution 

 is then used for development in tlie way already de- 

 scribed, and the fixing process immediately follows. 



Such is an outline of the earlier attempts of Mr. 

 Talbot ; which, however, have been modified by nume- 

 rous subsequent experimenters. The same principles 

 generally are involved, and the process is conducted 

 more rapidly, owing to the more sensitive nature of 

 the prepared paper. 



The calotype process, like many others, is not so 

 much in favour as that of the collodion, owing to the 

 numerous details which have to be observed, and the 

 tediousness of its manipulation. The effects, however, 

 are very pleasing ; and we may almost call it the most 

 elegant and refined of photographic processes, when it 

 is properly carried out. 



THE WAX-PAPER PROCESS. 



AMONGST other materials which have been used as a pho- 

 tographic material, we may briefly notice that of waxed 

 paper, which has the advantage of considerable sensi- 

 tiveness, and of affording a pleasing eti'ect when the 

 picture is complete. This, like all other photographic 

 processes, has been the subject of numerous modifica- 

 tions ; and we shall, therefore, only give a general out- 

 line of it. 



A thin kind of paper is to be soaked in the best melted 

 white wax, until that substance is well absorbed into its 

 -. The waxed paper is then placed between folds of 

 blotting-paper, and a hot iron is passed over the whole, 

 so that the wax may be equally diffused, and any super- 

 fluity bo removed. The paper, so prepared, is next 

 placed in a bath of iodide of potassium, to which a little 

 iodine has been added; and, after remaining some time, 

 it is carefully dried. It is sensitised by means of a 

 \.eak nitrate of silver bath, to which some acetic acid 

 has been added ; and if thus dried, it will keep for some 

 time, provided it has been well washed with distilled 

 water, to remove the superfluous nitrate from its surface. 



The length of exposure in the camera varies from one 

 miirute upwards ; and the latent picture is developed by 

 means of gallic acid and nitrate of silver. The fixing is 

 ed by a solution of the hyposulphite of sodium, 

 ami subsequent repeated washings in water. The pic- 

 ture may then be dried before a fire, and a hot iron 

 .should again be passed over it to equalise the wax in 

 each part of the paper. 



This plan ia not much employed at the present time ; 

 and, like most others, has been superseded by the collo- 

 dion process 



THE ALBUMEN PROCESS. 



A CONSIDERABLE variety of organic substances have been 

 tried by photographers, as media by which the sensitive 

 salts of silver may be retained on glass and paper sur- 

 faces. Amongst these, albumen has been frequently 

 employed ; and we have already drawn attention to one 

 of its uses when speaking of positive printing-paper, t 

 It has been applied with great success to glass plates; 

 and some of the productions of French artists are ex- 

 tremely beautiful. Strange to say, few of the pictures 

 produced exclusively by English photographers have 

 arrived at such excellence. 



Albumen, in this process, is used in place of collo- 

 dion. The manipulation, generally speaking, ia similar; 

 great precaution, however, being paid to the delicate 

 nature of the film, lest it should be injured. A picture 

 prepared thus has a very transparent appearance; and 

 the best of the kind we have seen are those of the blood 

 iles, to which reference has already been made. 

 Pictures by the albumen process are extremely valuable 

 for use in the oxy-hydrogcn microscope,^ having a con- 

 siderable transparency, and yet precision of detail. We 

 shall now give full directions for pursuing the collodio- 

 albumen process. 



t See ante, p. 153 



$ Sec ante, p. 63. 



