AMPHITYPE. At the York meeting of the British Association, in 1844, Sir John Herschel described a variety of the 'photo graphic' process to which he gave the name of Amphitype. The paper is saturated with one among several chemical solutions named by him ; and the paper so prepared and dried takes a negative picture, in a time varying from half an hour to five or six hours. The impression produced varies in apparent force, from a faint and hardly perceptible picture, to one of the highest conceivable fulness and richness both of tint and of detail : the colour being a rich soft brown. The pictures in this state are not permanent : they fade in the dark, though with different degrees of rapidity; but the picture is only dormant ; it may be restored, with a change of character from ne gative to positive, and of colour, from brown to black. This is effected by the employment of a solution of per-nitrate of mercury, into which the paper is steeped ; a long and care ful process of rinsing, drying, heating, and smoothing follows, by which a changed picture is produced, very much resembling the effect of a copper-plate engraving on a slightly yel low paper. It is from the production, by one and the same action of light, of either a posi tive or negative picture, according to the sub sequent manipulation, that the name of amphi type has been applied to the process. The
solutions in which the paper is originally steeped are any of the following,—ferro-citrate or ferro. nitrate of mercury: the metal being either the protoxide or the peroxide, or being superceded by the protoside of lead ; and the nitric acid being substituted for those above named. Steepings in these solutions are to alternate several times with steepings in solu tions of the ammonia-tartrate or ammonia citrate of iron.
Another curious process, which is an Amphi type on a different principle, also described by Sir John Herschel, depends on the steeping of a piece of paper, before the photographic process,in a solution of ferro-taxtrate of silver. If the paper, immediately on being removed from the photographic camera, be placed for a few seconds with its hack exposed to the sunshine, a positive picture, the exact com plement of the negative one on the other side, slowly and gradually makes its appear ance, and in half an hour or an hour acquires a considerable intensity, though unequal to the original picture on the other side in sharpness of detail. This is one of the most striking results in the whole range of photo graphy. [DAGUERROTYPE ; PHOTOGILintr, lee.]