470 PHOTOGRAPHY process were soon after discovered. The first consisted in more perfectly fixing the picture and deepening its shades, by the use of a salt of gold ; this was due to M. Fizeau. The second consisted in the use of a much more sensitive preparation, the silver bromide ; this diminish- ed the time of exposure in the camera to about one thirtieth of what was formerly required. The original process was modified in the iodi- zing part, the tablet being first exposed to iodine until it became yellow, then to bro- mine vapor arising from bromide of lime until a faint rose red was reached, and then back again to iodine vapor for a few moments. The other stages of the operation were conducted without any modification. As was shown by Dr. Draper in the paper referred to, and oth- ers subsequently published in the "Philosophi- cal Journal, no iodine is disengaged from the silver plate during the period of its exposure to light. The white portions of the resulting image consist of a compound of silver and mercury, a white amalgam of silver, while the shadows or dark parts are the pure silver un- changed. In an examination of some of these papers by Sir John Herschel, an opinion was expressed that the colors displayed by a da- guerreotype plate, and the peculiarity of its images, depend on the thickness of the film which has been affected ; but this opinion can scarcely be correct, since it is possible to copy a daguerreotype by electrotyping copper upon it, or even drying upon it a film of isinglass. The explanation given by Dr. Draper, that it is a dotted or stippled surface, the dots consisting of an amalgam of silver, is doubtless correct. Sir John Herschel communicated several elab- orate memoirs to the royal society, which were published in its " Transactions." These not only refer to the optical and chemical details of the subject, but also extend it to the case of new compounds, particularly the coloring ma- terials of flowers and plants. But the greatest improvement in the art of photography is due to Mr. F. Scott Archer of England, who dis- covered the collodion process. Of this the ad- vantages are so great, that the daguerreotype and calotype processes have become almost obsolete. It consists essentially in coating a clean glass plate with a solution of gun cotton in alcohol and ether, containing some soluble iodide. Very commonly the cadmium iodide is employed. After a momentary exposure to the air, the collodion is found adhering to the glass as a delicate film, the ether and alcohol having in part evaporated. The plate is now soaked in a solution of silver nitrate, technically called the nitrate bath, in which there must have been dissolved as much silver iodide as the so- lution will take up. Under these circumstances the iodides in the film become iodide of sil- ver. The glass is now transferred to the cam- era, enclosed in a screen to protect it from ex- traneous light. The exposure is then made as in daguerreotyping, and the invisible image is developed by pouring upon the film either a solution of pyrogallic acid or of protosulphate of iron. Too great activity in these substances is prevented by the previous addition of small quantities of acetic acid. The image comes forth as a negative, and it now remains to fix it. This is done by either soaking it in hypo- sulphite of soda, or pouring upon it a solu- tion of cyanide of potassium ; the film is then thoroughly washed with water and suffered to dry. From this negative proofs on paper may be printed, it having been first coated with amber varnish or some other suitable material that will not soften in the sun. But if the so- lutions used in its preparation have been much weaker than is necessary for the production of such a negative, and the quantity of iodide smaller, a positive on glass may be obtained in the first instance. Various names have been given to such positives, according to the man- ner of mounting them. Thus if the plate of glass bearing the image be joined to another plate by means of Canada balsam, and viewed against a black surface, it is designated an am- brotype. For the production of such positives a very much shorter time is required in the camera than for a negative. The collodion pro- cess is of two kinds, the wet and the dry. The former is that just described. In the latter the collodion film, after being carefully washed, is coated with some preservative material, such as tannin, and then dried. Though this plate is much less sensitive than the wet one, it is more convenient for certain purposes, such as the taking of landscape views. Sometimes, through inadequate exposure to light or a want of sensitiveness in the preparations, the result obtained as a negative is not sufficiently dense, and it becomes desirable to strengthen it in order to use it for printing. Various meth- ods have been recommended for this intensi- fying, as it is termed, but by far the best hith- erto published is that of Dr. Henry Draper, which simply consists in applying to the collo- dion picture before it is dry a solution of pal- ladium protochloride. This instantly produces an inky blackness in the dark parts, and affects in like manner the shades in the order of their gradation. It imparts no stain or impurity to the proof. The operation of printing from a negative is thus conducted. Paper of very uniform consistency is coated on one side with a thin deposit of albumen and silver chloride, conveniently produced by soaking the paper in ammonia chloride or chloride of sodium, and then laying it on the surface of a solution of nitrate of silver. Thus prepared, the paper is placed beneath a varnished negative, and exposed to the sun. The light transmitted through the glass in its transparent parts pro- duces blackness in the paper, but those places corresponding to the black portion of the neg- ative remain white in the proof, the inter- mediate shades being of course intermediately affected. When the change has taken place to a sufficient extent, the paper is removed from beneath the negative and soaked in a solution
