Page:The American Cyclopædia (1879) Volume XIII.djvu/485

 PHOTOGRAPHY 469 dergo a compression through the action of the prism itself. At the violet end they are wide- ly spread out, at the red they are concentra- ted. When a diffraction spectrum is used, this distortion does not occur, the colors being arranged in proportion to their wave lengths ; the heating power is then equal. When there- fore these peculiarities were duly considered, it was perceived that this hypothesis of the physical independence of light and heat was very far from having been established by these experiments. The first attempt at applying these principles for the delineation of exter- nal forms is to be attributed to Mr. Wedg- wood, who by imbuing leather with a solution of nitrate of silver, and exposing it under the images of a magic lantern slide, obtained what would now be termed negatives. Sir Humphry Davy made similar attempts; but as neither of these experimenters could fix the images thus obtained, their results were altogether abortive. About 1835 Prof. J. W. Draper, of the univer- sity of New York, began in the u Journal of the Franklin Institute " a series of papers on the subject. The facts investigated were chiefly in connection with the influence of light upon crystallization, the effect of colored absorbing solutions upon the chemical rays, and the in- terference and polarization of those rays. In his experiments, bromide of silver, and other compounds much more sensitive to light than any that had hitherto been used, were resorted to. In 1839 popular attention was suddenly directed to the subject by the announcement in France of Niepce and Daguerre's invention for the fixation of the images of the camera obscu- ra, and simultaneously in England of that of Mr. Talbot. In the former of these the material employed was a metallic tablet of silver-plated copper, in the latter paper. With these inven- tions the art of photography properly speaking begins. The process of Daguerre is as follows. A tablet of silver-plated copper is carefully cleaned, by means of pumice, rottenstone, or other suitable powders, from all adhering im- purity, and is brought to a perfectly reflecting and mirror-like surface. The success of the subsequent operations turns upon the purity and perfection of this surface. The tablet is then exposed to the vapor of iodine, rising at the ordinary temperature of the air, and in succession it passes through a series of bril- liant tints in the following order : pale lemon yellow, bright yellow, orange, red, blue, steel gray, clear metallic without color ; then again yellow, red, &c., in the same order. Of these tints the first and second yellow are the most sensitive to light, the others comparatively sluggish. The plate is therefore only exposed until the first full yellow is reached, and then with a careful exclusion of light it is deposited in the camera obscura, so as to receive the image. Here it remains for a period depen- dent on the brightness of the light, the length of which the operator learns from experience. Screened from the chance access of light, it is now removed from the camera, and if it be critically examined in a dark room by the light of a feeble taper, not the slightest change or action of any kind is perceptible upon it. Nevertheless there is an image concealed, which may be easily evoked by exposing the plate to the vapor of mercury at a temperature of about 170 F. After such an exposure for three or four minutes, the picture comes forth, the camera image being reproduced nearly in its proper order of light and shade. This ac- complished, it merely remains to dip the tablet in a solution of hyposulphite of soda, which instantly removes the yellow film or tarnish ; and after being copiously washed in clear water and dried, the photograph is insensible to any further action of light. In this operation of Daguerre's there are therefore several succes- sive stages: 1, the cleaning of the plate; 2, the iodizing ; 3, the exposure in the camera ; 4, mercurializing or development ; 5, fixing. These are terms which became of current use in the art. Mr. Talbot's invention of the calo- type or photogenic drawing, as he termed it, consisted essentially in covering a sheet of pa- per with a changeable salt of silver, exposing it in the camera, and developing the latent image by a solution of gallic acid. The re- sult was a negative ; that is, a photograph in which the lights and shadows answer respec- tively to the shadows and lights of the origi- nal ; while in a positive the lights correspond to lights, and the shadows to shadows. It had this advantage over Daguerre's, that it was capable of multiplication ; for from such a negative, if applied face downward on sen- sitive paper, many positive copies could be successively obtained by exposure to the sun. The daguerreotype, however, had a superior- ity unapproached even to this day by any oth- er process ; its images were exquisitely defined and sharp, and given with microscopic minute- ness. The reason of this superiority is obvious. The daguerreotype is formed on a mathemati- cal surface; the photograph in a translucent substance, in which the light can be diffused, and therefore the contours of objects are nev- er optically sharp. At first photography was limited to artificial views and interiors. It was found unsuited for the reproduction of land- scapes, the green color in which acts on the silver salts employed in a very sluggish way. The great and really valuable extension of its capabilities was that of taking portraits from the life. The initiation of this is due to Dr. Draper, who succeeded in it very shortly after Daguerre's process became known in Amer- ica, and who published the first complete ac- count of it in the "London and Edinburgh Philosophical Journal " of the following year. It is said that some of the portraits obtained by that chemist have not been since excelled. This great improvement was accomplished at a time when the inventor of the daguerreo- type himself had given it up as impracticable. Two other improvements on the daguerreotype