472 PHOTOGRAPHY ferences of opinion have prevailed respecting the relation between the principle thus in- volved and the luminous and calorific agen- cies. 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 allu- sion 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 per- fect 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 fol- lowed 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 investiga- tions, 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 found- ed 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 exten- sively employed in 1874 for recording the tran- sit of Venus. To the microscope it has like- wise been successfully applied, fixing the enor- mously magnified images presented by that instrument with a perfection and beauty alto- gether 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 oc- curring in living organisms. The reverse pro- cess of obtaining minute images, to be inspect- ed under the microscope, has been found use- ful 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 nee- dle, or of the index of the wind gauge, gives us trustworthy records of the temperature and pressure of the air, of the variations in ter- restrial 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 ammo- nia and dried in the dark. As soon as the film is dry the plate is placed with the film down- ward 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 be- comes indurated like leather, insoluble, and ad- hesive 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 de- scribed, 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 wash- ing 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 col- ored ink can be employed. All washing, to- ning, 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 copy- ing frame ; the sheet is pressed on pieces of India rubber, developed, and transferred ac- cording 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 suit- able holders, exposed, repacked in the hold- ers, and developed at home. The exposure re- quired is long, and the manipulation difficult,
