DAGUERROTYPE. 



433 



same manner as an ordinary copper-plate ; if this 

 be the case, the greatest secret still remains un- 

 known, even to M. Daguerre himself. It is much 

 to be regretted that M. Niepce did not at once 

 publish his extraordinary discovery, with a full de- 

 tail of the process employed, as he would then 

 have retained the indisputable right to the merit 

 of the invention. 



We must leave this point, and now proceed to 

 notice the claims of Mr Havell, in conjunction 

 with Mr Wellmore, who are said to have intro- 

 duced an important addition to the process pursued 

 by Mr Talbot, a full description of which is con- 

 tained in a letter to the editor of the Literary 

 Gazette. The first attempt of Mr Havell was 

 directed towards an etching, by Rembrandt, of an 

 old man reading, and the result was a reversed fac- 

 simile a negro face surmounted by locks of 

 silver ; the disappointed artist discovered that a 

 second transfer entirely destroyed the spirit of the 

 picture. To remedy this evil he had recourse to a 

 new process, by which this defect was indeed re- 

 moved, but the great merit of the art, namely, its 

 self-acting power, was lost. A thin plate of glass 

 was laid on the subject to be copied, upon which 

 the high lights were painted with a mixture of 

 white lead and copal varnish, the proportion of 

 varnish being increased for the darker shading of 

 the picture. The next day Mr Havell removed the 

 white ground with the point of a penknife, to re- 

 present the dark etched lines of the original, and 

 a sheet of prepared paper having been placed be- 

 hind the glass, and thus exposed to the light, a tol- 

 erable impression was produced ; the half tints had 

 however absorbed too much of the violet ray, an 

 imperfection which was remedied by painting the 

 parts over with black on the other side of the glass ; 

 if allowed to remain too long exposed to the sun's 

 rays the middle tints became too dark and destroyed 

 the effect of the sketch ; about ten minutes in 

 a powerful sun was considered sufficient. Another 

 method employed by Mr Havell was to spread a 

 ground composed of white lead, sugar of lead, and 

 copal varnish, over a plate of glass, and having 

 transferred a pencil drawing in the usual manner, 

 to work it out with the etching point till it bore 

 the appearance of a spirited ink drawing, or in the 

 hands of an engraver a highly finished engraving. 

 The above process Mr Havell made public under 

 the impression that it had been hitherto overlooked, 

 but Mr Talbot hearing that he was about to apply 

 for a patent, laid claim to the improvement as his 

 own, and not only pointed out some parts of his 

 former memorial where it was distinctly mentioned, 

 Imt also produced several drawings made precisely 

 in the manner described ; he also laid before the 

 royal society a new method of preparing the sensi- 

 tive paper, which consists in immersing it in a solu- 

 tion of nitrate of silver, and, after washing it with 

 bromite of potassium, the nitrate of silver is 

 again applied, the preparation being dried by the 

 fire between each operation ; the paper thus treated 

 is extremely sensitive, changing with the feeblest 

 daylight, first to a bluish green, then to olive green, 

 arid finally to black. 



A letter to Mr Talbot from his friend M. Biot 

 has also been published, and contains many inter- 

 esting experiments. After commenting upon the 

 value of the discovery, he continues " The in- 

 terest with which I viewed this circumstance en- 

 gaged me to make some experiments upon your 

 preparation, in order to vary its application to the 



VII. 



researches in which I am occupied. First I 



wished to know whether the change of colour was 

 in any degree influenced by the paper itself; I 

 therefore spread the substance on a piece of white 

 unglazed porcelain instead of paper, taking care to 

 operate by night, and drying it each time at the 

 fire, as you say : I thus obtained a dry solid coating 

 upon the porcelain, which I shut up in a dark place 

 until the morning. In the morning I took it out 

 and found it of a pale sulphur yellow colour ; I 

 then presented it to the daylight at an open 

 window looking north, the weather was then very 

 cloudy, yet no sooner had I so presented it than 

 already it was turned green, and soon afterwards 

 it became black. I then wished to know whether 

 the preparation would succeed equally well if not 

 dried at the fire; I therefore, in a darkened room, 

 mixed the aqueous solution of bromide of potassium 

 with that of nitrate of silver, a precipitate fell, 

 which I spread on a porcelain plate, and left it to 

 dry in the dark; the next day I wrapped it in sev- 

 eral folds of paper, and brought it into another 

 room to show it to a friend ; but having taken off 

 the covers in a dark corner of the room in order to 

 exhibit the original colour, pale lemon yellow, in- 

 stantly we saw its tint become green, and I had 

 hardly time to present it to a window opening to 

 the north before its colour had passed to dark olive 

 green, after which it almost immediately became 

 nearly black. I do not think it possible to find 

 any substance more sensitive to light." 



Meanwhile, the invention of M. Daguerre has 

 been purchased by the French government ; a 

 yearly pension of 6000 francs having been settled 

 on M. Daguerre, and another of 4000 francs on M. 

 Niepce, the son of the gentleman (deceased) by 

 whose experiments the invention was originally 

 suggested. On the 19th Aug. 1839, M. Arago 

 divulged the secret to a very crowded meeting of 

 the Academie des Sciences. M. Daguerre's pro- 

 cess is as follows: 



A plate of copper thinly coated with silver is 

 washed with a solution of nitric acid, for the pur- 

 pose of cleansing its surface, and especially to re- 

 move the minute traces of copper, which the layer 

 of silver may contain. This washing must be done 

 with the greatest care, attention, and regularity. 

 M. Daguerre has observed, that better results are 

 obtained from copper plated with silver, than from 

 pure silver ; whence it may be surmised, that elec- 

 tricity may be concerned in the action. 



After this preliminary preparation, the metallic 

 plate is exposed, in a well-closed box, to the action 

 of the vapour of iodine, with certain precautions. 

 A small quantity of iodine is placed at the bottom 

 of the box, with a thin gauze between it and the 

 plate, to sift the vapour, as it were, and to diffuse 

 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 quan- 

 tities 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 withdraw the 

 sheet of plated copper from the vapour, is indicated 

 ~>y 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 millimetre. The plate thus pre- 

 pared is placed in the dark chamber of the camera 

 obscura, and preserved with great care from the 

 aintest action of light. It is, in fact, so sensitive, 

 2* 



