The honor of having been the first to produce pictures by the action of light on is sensitive surface is now very generally conceded to Thomas Wedgwood, an account of whose researches was published in 1802 in the Journal of the Royal Institution, undn the title : "An account of a method of copying paintings upon glass, and of making pro files by the agency of light upon nitrate of silver; with observations by H. Davy." In die experiments detailed in this conununication, white paper and white leather were with nitrite of silver, and exposed either in the-camera obscura, or under the leaves of trees or wings of insects. The result was, that the shadows preserved the parts conceale:1 by them white, while the other parts became speedily darkened. The misfortune was. that no attempts made either by Wedgwood or Davy to prevent the uncolored portions from being acted on by light (or, as we now say, to fix the picture), were successful. This operation was not effected in a thoroughly efficient manner until sir John Herschel suggested the employment of bvposulphite of soda for that purpose. Many other fixing agents had been previously used, as ammonia, iodide of potassium, of sodium, and bromide of potassium, suggested by Mr. Fox Talbot; none of these, however, were found equal to the salt proposed and successfully used by sir John Herschel.
M. Niepce of Chalon on-the-Saone was the first to enjoy the satisfaction of producing permanent pictures by the influence of solar radiations. This was accomplished in 1314, and the name chosen to designate his process was helhigraphy—a name in some respects preferable to photography. It consisted in coating a piece of plated silver or glass with it varnish made by powdered asphaltum to saturation in oil of lavender, tak ing care that the drying and setting of this varnish be allowed. to take place in the .mire absence of light and moisture. The plate so prepared was then exposed. in the camera obscura for a length of time, varying from four to six hours! according to the amount of light. A faint image only is at first visible, and this is afterwards developed and fixed by immersion in a mixture of oil of lavender and nil of white petroleum; the plate being finally washed with water, and dried. Light has little or DO action on these heliographs; they should, however, be protected from moisture. M. Dagnerre improved on this process by suggesting the use of the resins obtained by evaporating oil of lavender, whereby a gnat increase of sensibility was secured.
Adopting date of publication as the best eviacnce of discovery, the next process offer ing itself for consideration is that for photogenic drawing by Mr. Henry Fox Talbot, communicated to the royal society on Jan. 31, 1839, previous to the pub lication of Dagnerre's process. It consisted in immersing selected writing-paper in a weak solution of common salt, and drying it. After this, a dilute solution of nitrate of silver was spread over one side, and the paper again dried at the fire. When dry it was fit for use, the sensitiveness being much increased by alternate treatment with saline and argentine solutions. Paper thus prepared yielded impressions in an incredibly short time, and nothing could be more perfoct than the images it gave of leaves and flowers, the light passing through the leaves delineating every ramification of their nerves. Con
siderable improvement in point of sensibility was attained by Mr. Talbot in the follow ing year, 1840, by the employment of iodide of silver on paper, as a foundation, to be washed over with a mixture of aceto-nitrate and gallo-nitrate of silver, just previous. to e_Tosure in the camera. Paper so prepared was so sensitive than an exposure of less than a second to diffused light was enough to produce an impression. After exposure and development, the picture was washed, and fixed by immersion in a solution of bro mide of potassium.
Niepce and Daguerre accidentally discovered that they were conducting experiments of a kindred character, and shortly afterwards entered into a partnership. The former, however, dying in July, 1833, a new deed of partnership was signed between his son Isi dore and M. Daguerre, which resulted in the publication, in July, 1839, of the process known as the daguerreotype. This was not done, however, until the French govern ment had passed a bill, to AI. Daguerre a pension of 6,000 francs, and to M..Isi dore Niepce, the son of the .Niepce, a pension of 4,000 francs, both for life, and one-half iu reversion to their widows. This handsome conduct on the part of the French govern ment was based upon the argument, that "the invention did not admit of being secured by Talent, since, as soon as published, all aught avail themselves of its advantages; they, there fore, chose to enjoy the glory of endowing the world of science and of art with one of the most surprising discoveries that honor their native land." The discovery of the daguerreotype may be said to have arisen from the lissatisfac don entertained by Daguerre with the insensibility of the bituminous surfaces of Niepce, which induced him to turn his attention to the salts of silver as a means of producing a higher degree of sensitiveness. This he attained by exposing a highly polished plate of silver (attached, for greater strength, to a copper plate) to the vapor of iodine, by which pure iodide of silver was formed on the surface.. The plate so prepared was exposed in the camera obscura for a length of time (20 minutes), which was then considered very short. No apparent effect was produced on the plate, the image being a latent one, aris ing from a minute molecular disturbance caused by the impact of the actinic rays. The latent image was afterwards developed by exposing the plate to the vapor of mercury; and it is this development of a latent image, reducing as it did the time of exposure from hours to minutes, which truly constituted a new era in the science of photography. It is further due to Daguerre to state, that, while his processes for the purpose were imper fect, he still succeeded in fixing his pictures, although it was reserved for sir John Her schel to announce the great suitability of the hyposulphites for dissolving the haloid salts of silver. ,The sensibility of the silver plate was still further increased by Mr. Goddard, who suggested, in 1839, the association of the vapor of bromine with that of iodine; while M. Claudet, in 1840, employed chlorine. It is a remarkable fact, in connection with these discoveries, that the elder Niepce. should, so early as 1820, have tried the treatment of silver plates with the vapors of sulphur and phosphorus.