Photography (Gr. Photography 1300458 , light, and Photography 1300459 , to write), the art of depicting objects by the agency of light. The earliest observations on the chemical changes produced by the agency of light were doubtless those of the fading and bleaching of vegetable colors. Of the delicate tints that may be obtained from the parts of plants, some are so sensitive to light that an exposure of only a few moments is sufficient to injure them; others resist for a longer time the conjoint action of sunlight, air, and water. It was noticed by the alchemists, probably about the 12th century, that the chloride of silver blackens by exposure to the sun, though when first prepared it is as white as snow. This darkening is in a general manner proportional to the brightness of the light. It does not occur instantaneously, but in a regulated way, a given quantity of light being apparently necessary for the production of a definite effect. As experimental chemistry was cultivated, the list of substances thus influenced became greatly extended, and when Berzelius published his work on chemistry several scores of bodies were known to be changeable by luminous agency. Some of these were elementary, and some were compounds, derived from both the inorganic and organic groups.

Perhaps the first germ of photography as an art is presented in an experiment of Priestley's, who caused some chloride of silver to be deposited on the side of a glass bottle, and then putting round the bottle a piece of dark paper out of which letters had been cut with a penknife, the arrangement was exposed to the sun. All those portions of chloride upon which the light had fallen, through the spaces where the paper had been removed, turned black, but those protected by the dark paper retained their whiteness unimpaired. Scheele also made some very instructive experiments for the purpose of determining whether it was some specially colored ray of light, or light in the aggregate, that produced the result. He caused a beam to enter a darkened chamber through a hole in the window shutter, as in Newton's experiment for the decomposition of light, and, intercepting the beam by means of a glass prism, dispersed it into its constituent rays. The colored spectrum thus produced was received on a sheet of paper painted over with chloride of silver. The blackening began in the indigo or violet region, and extended in the more refrangible direction far beyond the limits of visibility.

In the other direction it stopped short in the blue space, so that the green, the yellow, the orange, and the red exhibited no kind of action. From this it appears that a sunbeam does not darken the chloride of silver in virtue of its light, but that the decomposition is brought about by some other principle contained in the beam, conjoined with the light, and found to the greatest degree in the more refrangible end of the spectrum. To the rays thus recognized as occasioning the changes the designation of chemical rays was given, and they were likewise called deoxidizing rays. In observations upon the sun with reflecting telescopes, Sir William Herschel had been obliged to use colored glass screens, for the purpose of diminishing the excessive brilliancy of the light, and had accidentally noticed that the heat transmitted through these colored glasses was very far from being proportional to the light. A glass colored deeply enough to absorb a large portion of the light rays might nevertheless transmit an unexpected proportion of the heat rays. He therefore prepared a solar spectrum after the manner of Newton, and set in each of its colored spaces the bulb of a delicate thermometer.

Starting from the violet and descending through the indigo, the blue, green, yellow, orange, and red, the thermometer stood higher and higher; outside of the red and beyond the visible limits of the light, it stood highest of all. No other interpretation could apparently be given to such an experiment than that the heat and the light are altogether independent agents, and distributed very differently in the spectrum. This conclusion contained, however, a most important error, which was perpetuated until a later period in these discoveries; it overlooked the physiological peculiarities of an organ of vision like the human eye. It also overlooked the physical fact that the colored spaces from the violet to the red undergo a compression through the action of the prism itself. At the violet end they are widely spread out, at the red they are concentrated. 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 therefore 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 external forms is to be attributed to Mr. Wedgwood, 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 university of New York, began in the " 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 interference 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 inventions 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 impurity, 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 brilliant tints in the following order: pale lemon yellow, bright yellow, orange, red, blue, steel gray, clear metallic without color; then again yellow, red, etc, 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 dependent 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 accomplished, 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 successive 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 paper with a changeable salt of silver, exposing it in the camera, and developing the latent image by a solution of gallic acid. The result was a negative; that is, a photograph in which the lights and shadows answer respectively to the shadows and lights of the original; 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 sensitive paper, many positive copies could be successively obtained by exposure to the sun. The daguerreotype, however, had a superiority unapproached even to this day by any other process; its images were exquisitely defined and sharp, and given with microscopic minuteness. The reason of this superiority is obvious.

The daguerreotype is formed on a mathematical surface; the photograph in a translucent substance, in which the light can be diffused, and therefore the contours of objects are never optically sharp. - At first photography was limited to artificial views and interiors. It was found unsuited for the reproduction of landscapes, 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 America, and who published the first complete account 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 daguerreotype himself had given it up as impracticable. Two other improvements on the daguerreotype 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 H. Fizeau. The second consisted in the use of a much more sensitive preparation, the silver bromide; this diminished the time of exposure in the camera to about one thirtieth of what was formerly required. The original process was modified in the iodizing part, the tablet being first exposed to iodine until it became yellow, then to bromine 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 others subsequently published in the " Philosophical Journal,'1 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 unchanged.

In an examination of some of these papers by Sir John Herschel, an opinion was expressed that the colors displayed by a daguerreotype 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 elaborate 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 materials of flowers and plants. - But the greatest improvement in the art of photography is due to Mr. F. Scott Archer of England, who discovered the collodion process. Of this the advantages 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 solution will take up'. Under these circumstances the iodides in the film become iodide of silver. The glass is now transferred to the camera, enclosed in a screen to protect it from extraneous 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 hyposulphite of soda, or pouring upon it a solution 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 solutions 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 manner 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 process 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 methods have been recommended for this intensifying, as it is termed, but by far the best hitherto published is that of Dr. Henry Draper, which simply consists in applying to the collodion picture before it is dry a solution of palladium 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 produces blackness in the paper, but those places corresponding to the black portion of the negative remain white in the proof, the intermediate 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 of hyposulphite of soda.

This dissolves out all the unaffected silver chloride, and leaves the picture without liability to further change. But as the tone or tint of color that it presents is commonly regarded as unpleasing to the eye, it is laid in a bath containing chloride of gold, which after a while imparts to it a delicate violet hue. Toning baths, as they are termed, of various ingredients, and capable of imparting shades of a sepia and brown tint, are recommended by different operators. They are too numerous to be here described. - The following formulas for the collodion process have been recommended: 1. For the collodion: gun cotton, 4 to 8 grains; sulphuric ether, sp. gr. .720, 5 fluid drachms; alcohol, sp. gr. .825, 3 fluid drachms; iodide of cadmium, 4 to 5 grains. 2. For the nitrate bath: water, 1 fluid ounce; nitrate of silver, 30 grains; as much iodide of silver as it will dissolve. 3. For the developer: water, 1 fluid ounce; pyrogallic acid, 1 grain; acetic acid, 10 to 20 minims. 4. Or this: water, 1 fluid ounce; protosulphate of iron, 12 to 20 grains; acetic acid, 20 minims. 5. For the fixing solution: water, 1 fluid ounce; cyanide of potassium, 2 to 20 grains. 6. For the fixing solution (another formula): water, 1 fluid ounce; hyposulphite of soda, 1/2 ounce.

The following formulas may be useful in the printing process: 7. For the salting solution: chloride of ammonium, 200 grains; water, 10 ounces. 8. For the sensitizing solution: nitrate of silver, 60 grains; water, 1 fluid ounce. 9. For the fixing solution: hyposulphite of soda, 4 ounces; water, 8 ounces. 10. For the toning solution: chloride of gold, 4 grains; hyposulphite of soda, 4 ounces; water, 8 ounces. If the paper previously to being sensitized has been imbued with albumen, the resulting proofs (albumen proofs, as they are termed) have a glossy and much improved appearance. It is to be understood that a print as well as the original collodion must be thoroughly washed in clear water after the process for fixing or toning has been completed; otherwise it will be liable to a spontaneous fading away. - Among the applications of photography must not be omitted the interesting one of the stereoscope. Stereoscopic photographs may either be made by a purposely constructed camera with a pair of lenses, or by a single camera set successively in two different determinate positions.

The illusion of the stereoscope gathers force from the truth of the photograph, and such pictures, from the air of solidity that they present, give a very striking result, not only in the case of portraits from the life, but also in landscapes, and especially in architectural objects. - Though photographs as now produced by the best artists are of very great beauty, they are nevertheless very imperfect. They do not critically represent the exact order of light and shade; and what is a still greater defect, they do not represent the order of luminosity, as dependent upon the coloration of the object. To the eye the yellow is the brightest color, the intensity of the light declining as we go to the violet end of the spectrum on one side, and to the red on the other. But in all the silver preparations in use among photographers, the indigo ray produces the greatest effect, and therefore may be said to possess the greatest illuminating power, and from it the intensity declines toward the violet on one side, and ceases on the other before the yellow is reached. The effect of this in a photograph may be easily understood.

If two pieces of paper, one painted light yellow and the other deep indigo, be examined by the eye, the former impresses us most vigorously, and we speak of it as being bright in comparison with the other. But if a photograph of these two pieces of paper be taken, the deep indigo will come out white, and the light yellow completely black. So the real order of their visual intensity is reversed in their photographic representation. For this reason photography has given unsatisfactory results in its application to landscapes. - Nothing would tend so quickly to the improvement of photography as the invention of some means for the accurate admeasurement of light; i. e., some instrument that would answer for the luminous agent, as the thermometer answers for heat. The contrivances that have been recommended may answer well enough in the hands of an accomplished chemist, but are unsuitable for the common operator. Among them may be mentioned the galvano-photome-ter of M. Becquerel, the chlorine and hydrogen photometer of Dr. Draper, the photometer of Bunsen, and the test paper of Roscoe. - Owing to the want of durability of photographs obtained by the aid of salts of silver, attempts have been made to substitute for those compounds others not liable to change.

Among such may be classed carbon, which is altogether unalterable in the air. The carbon process has now reached a degree of perfection that will bring it into competition with the older methods. Improvements have also been made in the operation of development; that known as the alkaline development gives very satisfactory results. Recently it has been announced by Prof. Vogel that the sensitiveness of dry plates is increased by adding to the collodion certain coloring matters, such as fuchsine and chlorophyl, which impart a sensitiveness to the green and yellow rays. Promising attempts have been made by Col. Stuart Wortley, Mr. Carey Lea, and others, to get rid of the use of the nitrate bath, by covering the plates with an emulsion containing a salt of silver instead of with iodized collodion. This is the emulsion or collodio-bromide process. - From what has been said respecting visual impressions and photographic representations, it will be perceived that the agent which accomplishes the latter is not light, and that therefore the term photography is in truth a misnomer. Among those chemists who have examined the scientific connections of this subject, differences of opinion have prevailed respecting the relation between the principle thus involved and the luminous and calorific agencies.

These differences of opinion have led to different designations for the rays darkening the silver preparations. Some of the earlier experimenters spoke of them as deoxidizing rays, some as chemical rays; others, in allusion to their position at the more refrangible end of the spectrum, as violet rays; others as tithonic rays; but the term that has met with most general acceptance is actinism or actinic rays. (See Actinism, and Light.) - Optical instruments for photographic purposes must be of the most perfect kind, and provided with the necessary means for depicting a perfect image of the objects to which they are directed. They must therefore be not only achromatic in the common acceptation of that term, but also achromatic photographically, and likewise have adequate provisions against spherical aberration. - Photography is now followed as an industrial pursuit in the United States and other countries by many thousands of persons. It also possesses a very extensive literature, from the highest scientific investigations, inserted in the transactions of various learned societies and special treatises in many instances of considerable size, to periodicals. Besides those who practise it professionally, it finds amateurs everywhere.

In the United States, England, France, and Germany, photo-graphical societies are well sustained in many of the large towns. The general popularity which photography has thus attained is founded not only upon the realized perfection with which it can perpetuate external forms, but also upon the anticipated advantages to accrue from it in several of the higher departments of science. It has been employed for the purpose of permanently recording the aspect of the moon and the eclipses of the sun, and has given us representations of the planets, and even the configuration of the stars. It was very extensively employed in 1874 for recording the transit of Venus. To the microscope it has likewise been successfully applied, fixing the enormously magnified images presented by that instrument with a perfection and beauty altogether unattainable by the hand of man. In this manner questions of the utmost importance in physiology and the sciences of organization, which have long been in dispute, have received a final solution, and permanent representations have been obtained of transient phenomena occurring in living organisms. The reverse process of obtaining minute images, to be inspected under the microscope, has been found useful for various purposes.

During the siege of Paris (1870-'71) letters were thus reduced for transmission by carrier pigeons, and four pages of the London "Times" were reproduced on a sheet of 3 by 5 in. In meteorology likewise, a coil of sensitive paper receiving the shadow of the mercury in the thermometer or in the barometer, or of the suspended magnetic needle, or of the index of the wind gauge, gives us trustworthy records of the temperature and pressure of the air, of the variations in terrestrial magnetism, and of the motions of the atmosphere. - The albertype is a picture in printer's ink, made by the lithographic press. To work the process, a thin, perfect negative is taken, which must not be varnished; this is coated with collodion, and when the film is dry a cut is made through it to the glass all round the edge. The plate is next soaked in water, and the film soon becomes loosened so that by raising one corner with the point of a knife it may be easily removed from the glass. It is dried and placed between the leaves of a blank book, and becomes the future negative. A piece of plate glass about half an inch thick, properly ground and polished, is coated with albumen saturated with bichromate of ammonia and dried in the dark.

As soon as the film is dry the plate is placed with the film downward on a piece of black velvet, supported on a flat board. In this condition it is carried into the light, and exposed in such a way that the light has to pass through the plate glass and act upon the back of the film, which becomes indurated like leather, insoluble, and adhesive to the glass, while the front surface of the film, by reason of the black velvet, remains soluble and can be removed by washing to make room for the next coating. This coating consists of the finest gelatine and isinglass, sensitized with bichromate of ammonia. The thin negative film, prepared as previously described, is now placed face upward upon the sensitized gelatine film of the glass plate and exposed in a suitable printing frame. The bi-chromated gelatine, after exposure and washing in water, becomes the type plate, which is imbedded in gypsum on a marble slab previous to inking and printing. An indefinite number of impressions Can be taken in this way, to keep as many presses in operation as may be required. One operator attending to one press can produce about 200 prints in a day. A graded border, with title, description, and date, can be printed at the same time, and any colored ink can be employed.

All washing, toning, fixing, and mounting is dispensed with, and there is no danger of the print's fading, as the ink is carbon. - Pigment or carbon prints are made upon paper covered with gelatine. The gelatine is sensitized with bichromate of potash and exposed under a negative in a copying frame; the sheet is pressed on pieces of India rubber, developed, and transferred according to a method invented by Swan. As the gelatine may be colored any shade, perfect copies of old tracings, etchings, and drawings can be made, and there is no danger of the picture's fading. A dry tannin process has been invented by which plates are prepared in the laboratory, taken on journeys in suitable holders, exposed, repacked in the holders, and developed at home. The exposure required is long, and the manipulation difficult, so that as a general rule photographers prefer the wet silver process, as they can at once see what they are about, and if the negative is not good can make a second trial. Woodbury's photo-relief process consists in preparing a gelatine print and transferring this to soft metal by a hydraulic press. The raised parts of the gelatine film are forced into the soft metal, thus giving a picture with the light and shadows reversed.

A hot mixture of gelatine, with some coloring substance, is poured upon the intaglio plate, a sheet of well sized paper is then laid on, and the whole is pressed; as soon as the gelatine is cool, the impression is removed and further fixed by alum and tannin. Osborne's photo-lithographic process is still another adaptation of bichromated gelatine. (See Lithography, vol. x., p. 539.) - On the fine arts the effect of photography has been important, and every day is increasing the number of its applications to artistic purposes. Soon after the discovery of the collodion process its facilities for the multiplication of copies attracted the attention of publishers. Mr. G. P. Putnam of New York was the first to introduce it practically by the insertion of some photographic views in the " Homes of American Authors" (New York, 1852). In France and Germany many splendid books have been illustrated by the same method, as the superb memorial edition of Schiller with photographs from the original drawings of eminent living artists of Germany. For the representation of scenery or the copying of old paintings, drawings, etc, it is equally available.

The most ancient Greek manuscript of the New Testament, discovered at Mount Sinai by Tischen-dorf, was photographed in four large volumes folio, at the expense of the emperor of Russia; and the British government has perpetuated by the same process the "Domesday Book." On the art of painting, the unlimited supply of photographic studies, combining breadth of effect with nature's own minuteness of detail, is producing the most marked results; while, should the progress of photography keep pace with its past achievements, many of the laborious and expensive enterprises of engraving on steel and copper will soon be entirely superseded. - See " The Amateur's Photographic Guide Book," by W. J. Stillman (London, 1874), and "The Chemistry of Light and Photography," by Hermann Vogel (English translation, New York, 1875).