264 INDUCTION AND DEDUCTION. 



l^laces become, in proportion to the strengtli of the liglit, blackened into 

 corres])Oudiug- shadows ; the obscure places remain white or luminous. 

 The sashes of a window, for instance, throw less light on the paper than 

 the glass panes ; a dark stone less than the bright stones. ' Whatever 

 is dark in the object appears bright 5 the bright, dark. There is im- 

 pressed on the paper a so-called negative image. If the paper be now 

 washed with a solution of sub-sulphurated natron, so much of the chlo- 

 ride of silver as is unchanged by the light is removed ; had this remained 

 on the paper the image would by degrees become black, nnder the ope- 

 ration of the light, and again disappear. Tlie salt mentioned is there- 

 fore the medium through which it becomes fixed. 



The first images presented by Talbot were very imperfect ; as their 

 production required a long exposure to the operation of the light, only 

 the images of perfectly immovable objects could be obtained. The ex- 

 periments of Daguerre gave occasion to the perfecting of Talbot's pro- 

 cedure, but in a singular manner. Daguerre exposed his silvered plates 

 to the influence of the vapor of iodine, and in this way gave them an 

 extremely thin coating of silver of iodine ; but from this resulted no 

 image in the camera obscura. Months of trials, varied in all manner of 

 ways, afiorded no results. At length hazard, in the most proper sense, 

 came to his aid ; Daguerre had x)ut aside a number of the plates which 

 had served for his experiments in an old press, where they remained for 

 weeks without further attention. Happening on some occasion to take 

 one of them out, he saw, to his utmost surprise, an image traced upon 

 it, of the greatest distinctness to the minutest particulars. No idea had 

 he how it was produced, but, of course, there must be something in the 

 press which had brought to light the image on the plate. Now, there 

 were all sorts of things therein : Utensils, apparatus, chemical reagents, 

 and, among the rest, a vessel containing quicksilver. Daguerre pro- 

 ceeded to remove one article after another, overlooking, however, the 

 quicksilver; and he still procured images whenever he allowed one of 

 the plates, on which he had thrown an image in the camera obscura, to 

 remain some two hours in the press. Of the quicksilver he did not 

 think ; the old press had begun to appear to him an enchanted press. 

 At last it occurred to him that the image must proceed from the quick- 

 silver, and it turned out to be what may be called a breath picture. If 

 a drawing be made with a wooden pencil on a clean glass plate, the 

 sharpest sight will fail to discover the lines, which nevertheless become 

 distinguishable when the places marked by the pencil are breathed upon. 

 There exists, in effect, in these places and other parts of the glass an 

 unequal condensation of the watery vapor which is deposited thereon in 

 fine drops. 



In this manner Daguerre's images originated. Quicksilver is volatile, 

 and its vapor had been diffused through the press and settled in minute 

 drops on the plates, taking effect more strongly on the more illuminated 

 l^arts, so that the outlines and shadings of all objects were rendered 



