PHOTOGRAPHY, or HELIOGRAPHY. Under the article Daguerreotype, full men tion of the action of the salts of silver un der the influence of light has been de scribed. Photographic processes require no silver plate, making use of paper or some non-conducting material and apply ing on its surface sensitive salts of silver, which are to be protected from the light until they are ready to be exposed in the camera.
The term " Photogenic Drawing" has usually been applied to representations of various objects upon paper imbued with some of the salts of silver. If a piece of paper be dipped into a weak solution of nitrate of silver, carefully dried, and pre served out of the contact of light, it re mains white ; but if exposed to light it gradually becomes discolored, acquiring a brownish or gray tint, and ultimately blackens, the depth of color depending upon the intensity of the light and dura tion of exposure. If any opaque or trans lucent object be laid upon a sheet of pa per so prepared, so as wholly or partially to intercept the incident light, a represen tation of the object is obtained upon the paper. Where the light has been wholly intercepted, it remains white ; where par tially so, various shades are produced ; and wherever the light has fallen without interruption, the utmost blackness is ob tained. If, for instance, a portrait paint ed in transparent colors upon a plate of glass be laid upon a piece of the prepared paper, and exposed to the solar light, a copy is obtained in which the lights of the original are shades, and the shades lights in proportion to their intensity ; but if such a picture be taken upon a very thin piece of paper, this may be again copied by a repetition of the process, and then the lights and shades will be as in the ori ginal. It is, however, obvious that such a photograph will only be durable whilst kept in the dark, and that exposure to light will gradually obliterate the whole ; to fix it, the paper must be washed in a solution of hyposulphite of lime or of so da, which removes all remaining and un altered salt of silver, but leaves the image untouched. In this process the paper, after having been impregnated with ni trate of silver, or with ammonia-nitrate of silver, is generally dipped in a solution of common-salt, by which chloride of silver is formed, and this is more susceptible of the influence of light than the mere ni trate.
Various salts of silver have been used by different operators, and the processes .have received. different names : those of Mr. Talbot and Sir. J. Herschel are the most approved. The invention was first made public by M. Arago. Mr. Hunt's process called chromatype is given under that article, and under the head calotype is given Mr. F. Talbot's process.
The Cyanotype of Sir J. Herschel is made by washing the paper with a solu tion of ammonio citrate of iron. It is then exposed to light, and a latent picture impressed on it. If the paper be sensibly darkened, the picture will appear nega tive. It is now touched over sparingly and equally with a solution of ferro-cya nide of potassium in which is dissolved a littlegum. The negative picture vanish es and is replaced by a positive one of a violet blue on a green ground ; a second washing brings out the picture clearer.
A second process of the cyanotype is, to saturate the paper with a solution of equal parts of ammonia citrate of iron and ferro-Sesquicyanide of potassium. When a picture has been impressed it is thrown into water and then dried, and a negative picture results. When this is washed with solution of proto-nitrate of mercury it is discharged, but may be restored by wash. ing out the nitrate and drying the paper. A smooth hot iron is now passed over it, and the obliterated picture comes out of a brown tint • these photographs fade, but are restored by heat.
Third process : 1 pint of ammonio-ci trate of iron is dissolved in 11 parts of water, and this is mixed with an equal quantity of a cold, saturated solution, of bichloride of mercury, before the precipi tate is formed : the solution is brushed over paper, which should have a tint of yellow. This paper keeps well: when a picture is formed on it, it is washed over with a saturated solution of prnssiate of potash, diluted with thin gum water. The picture is fixed by drying, and are beauti ful positive ones.