1S39.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



321 



THE DAGUERROTYPE. 



The secret process of this important discovery for producing Pho- 

 tographic plates, was revealed to the French public at the setting of 

 the " Academie des Sciences," on Monday, 19th ultimo ; the attendance 

 at the meeting was very numerous, so much so, that upwards of 2(J0 

 persons were unable to obtain admittance. M. Arago commenced his 

 observations by referring to the process of M. Niepce, the first dis- 

 coverer of the art, and the subsequent improvements made by M. 

 Daguerre. He then proceeded to explain the great discovery, of 

 which the following is an extract from a report in the Journal dts 

 Dibals. 



M. Arago stated that, according to M. Daguerre's process, copper 

 plated with silver is washed witli a solution of nitric acid, for the 

 purpose of cleansing its surface, and especially to remove the minute 

 traces of copper, which the layer of silver may contain. This wash- 

 ing must be done with the greatest care, attention, and minuteness. 

 M. Daguerre has observed, that better results are obtained from cojiper 

 plated with silver, than from pure silver; whence it may be surmised, 

 Arago observes, that voltaic action is coimected with this pheno- 

 menon. 



After this first, preparation, the metallic plate is exposed, in a well- 

 closed box, to the action of the vapour of iodine, with certain precau- 

 tions. A small quantity of iodine is placed at the bottom of the box, 

 with a thin gauze between it and the plate, as it were, to sift the 

 vapour, and to diHuse it equally. It is also necessary to surround the 

 plate with a small metallic frame, to prevent the vapour of iodine from 

 condensing in larger quantities round the margin than in the centre; 

 the whole success of the operation depending on the perfect uniformity 

 of the layer of ioduret of silver thus formed. The exact time to with- 

 draw the sheet of plated copper from the vapour, is indicated by the 

 plate assuming a yellow colour. M. Dumas, who has endeavoured to 

 ascertain the thickness of this deposit, states that it cannot be more 

 than the millionth part of a millimetrt. The plate thus prepared, is 

 placed in the dark chamber of the camera obscura, and preserved with 

 great care from the faintest action of light. It is, in fact, so sensitive, 

 that ex]josure for a tenth of a second is more than sufficient to make 

 an impression on it. 



At the bottom of the dark chamber, which M. Daguerre has reduced 

 to small dimensions, is a piate of ground glass, which advances or re- 

 cedes until the image of the object to be represented is perfectly clear 

 and distinct. When this is gained, the prepared plate is substituted 

 for the ground glass, and receives the impression of the object. The 

 effect is produced in a very short time. When the metallic plate is 

 withdrawn, the impression is hardly to be seen, the action of a second 

 vapour being necessary to bring it out distinctly: the vapour of mer- 

 cury is employed for this purpose. It is remarkable, that the metallic 

 plate, to be properly acted upon by the mercurial vapour, must be 

 placed at a certain angle. To this end, it is enclosed in a third box, 

 at the bottom of which is placed a small dish filled with mercury. If 

 the picture is to be viewed in a vertical position, as is usually the case 

 with engravings, it must receive the vapour of mercury at an angle of 

 about 45°. If, on the contrary, it is to be viewed at that angle, the 

 plate must be arranged in the box in a horizontal position. The vola- 

 tilization of the mercury must be assisted by a temperature of 6U° of 

 Reaumur = 11J7" F. 



After these three operations, for the completion of the process, the 

 plate must be plunged into a solution of hypo-sulphite of soda. This 

 solution acts most strongly on the parts which have been uninfluenced 

 by light; the reverse of the mercurial vapour, which attacks exclu- 

 sively that portion which has been acted on by the rays of light. 

 From this it might perhaps be imagined, that the lights are formed by 

 the amalgamation of tlie silver with mercury, and the shadows Ijy the 

 sulphuret of silver formed by the hypo-sulphite. M. Arago, however, 

 formally declared the positive inability of the combined wisdom of 

 physical, chemical, and optical science, to offer any theory of these 

 delicate and complicated operations, which might be even tolerably 

 rational and satisfactory. 



The picture now produced is washed in distilled water, to give it 

 that stability which is necessary to its bearing exposure to hght with- 

 out undergoing any further change. 



After his st.itement of the details of M. Daguerre's discovery, M. 

 Arago proceeded to speculate upon the improvements of which this 

 beautiful a]iplication of optics was capable. He adverted to M. 

 Daguerre's hopes of discovering some further method of fixing not 

 merely the images of things, but also of their colours : a hope based 

 upon the fact, that, in the experiments which have been made with 

 the solar spectrum, blue colour has been seen to result from blue rays, 

 orange colour from orange, and so on with the others. Sir John Her- 

 schel is sure that the red ray alone is without action. The question 



arose, too, %vhether it will be possible to take portraits by this method ? 

 M. Arago was disposed to answer in the affirmative. A serious diffi- 

 culty, however, presented itself: — entire absence of motion on the 

 part of the object is essential to the success of the operation, and this 

 is impossible to be obtained from any face exposed to the influence of 

 so intense a light. M. Daguerre, however, believes that the interposi- 

 tion of a blue glass would in no way interfere with the action of the 

 light on the prepared plate, while it would protect the sitter suffi- 

 ciently from the action of the light. The head could be easily fixed 

 by means of supporting apparatus. Another more important desitler- 

 atum is, the means of rendering the picture unalterable by friction. 

 The substance of the pictures executed by the Daguerrotype is, in 

 fact, so little solid — is so slightly deposited on the surface of the me- 

 tallic plate, that the least friction destroys it, like a drawing in chalk : 

 at present, it is necessary to cover it with glass. 



From his numerous experiments on the action of light on different 

 substances, M. Daguerre has drawn the conclusion that the sun is not 

 equally powerfully at all times of the day, even at those instants when 

 his height is the same above the horizon. Thus, more satisfactory 

 results are obtained at ten in the morning than at two in the afternoon. 

 From this, too, it is evident, that the Daguerrotype is an instrument 

 of exquisite sensibility for measuring the diiferent insensities of light, 

 a subject which has hitherto been one of the most difficult problems 

 in Natural Philosophy. It is e;isy enough to measure the difference 

 in intensity between two lights viewed simultaneously, but when it is 

 desired to compare daylight with a light produced in the night — that 

 of the sun with that of the moon, for example — the results obtained 

 have had no precision. Tlie preparation of M. Daguerre is influenced 

 even by the light of the moon, to which all the preparations hitherto 

 tried were insensible, even when the rays were concentrated by a 

 powerful lens. 



In physics, M. Arago indicated some of the more immediate appli- 

 cations of the Daguerrotype, independently of those which lie had 

 already mentioned in Photometry. He instanced some of the most 

 complex phenomena exhibited by the solar spectrum. We know, for 

 example, that the difl'erent coloured rays are separated by black tran- 

 versal lines, indicating the absence of these rays at certain parts ; ami 

 the question arises whether there are also similar interruptions in the 

 continuity of the chemical rays ? M. Arago proposes as a simple 

 solution of this question, to expose one of M. Daguerre's prepared 

 plates to the action of a spectrum ; an experiment which would prove 

 whether the action of these rays is continuous or interrupted by blank 

 spaces. 



The description of the process appeared to excite great interest in 

 the auditory, amongst whom we observed many distinguished persons 

 connected with science and the fine arts. Unfortunately the locality 

 was not adjudged suitable for the ]jerformance of M. Daguerre's ex- 

 periments, but we understand that arrangements will be made for a 

 public exhibition of them. Three highly curious drawings obtained 

 in this manner were exhibited ; one of the Pont Marie ; another of 

 the M. Daguerre's atelier ; and a third of a room containing some rich 

 carpeting, all the minutest threads of which were represented with 

 the most mathematical accuracy, and with wonderful richness of eftect. 



MORE GOVERNMENT JOBBING. 



The tocsin of alarm has been sounded by an influential Whig mar- 

 quis for another Government enquiry into the causes of accidents on 

 railways; and doubtless an attempt will be made to issue a commis- 

 sion similar to that on "Steam Vessel Accidents," which we noticed 

 very fully in our last number. No doubt the Commission will be issued 

 with the sinister understanding that a similar report is to be recom- 

 mended, for the appointment of Government Inspectors, to superintend 

 the working and coi struction of railways, and that no locomotive en- 

 o-ine shall be allowed to run without a license from these Obstructors. 



It behoves all manufacturers to unite and act firmly in resisting these 

 encroachments on British enterprise, for if Government once can obtain 

 an act for such a purpose, it will be carried to other branches of our 

 Manufactures and commerce. 



The Railway Companies have already united themselves into an 

 Association, and thereby resisted very serious encroachments attempted 

 to be made on their rights; a similar association we again advise 

 ought at once to be formed by the Steam Boat proprietors and builders, 

 fur if they leave it till next Session of Parliament, they may depend 

 upon it, that attempts will be made to levy such arbitrary laws and 

 restrictions on their proceedings and profits, that they will not be able, 

 without considerable difficulty, to get them again repealed or altered. 



2 A 



