2S8 Photographic Processes. 



picture. But, whatever may be the details of the process em- 

 ployed, a platinotype slowly vanishes, even in the dark — 

 though, in some cases, it gradually reappears. 



The Catylissotype. Paper is brushed over with a mixture 

 consisting of syrup of iodide of iron and tincture of iodine, 

 and, when dried with blotting paper, is washed with nitrate 

 of silver. After exposure, nothing is perceptible ; but a pic- 

 ture gradually developes in the dark. The name of the process 

 is due to the supposition that, when the silver salt has been 

 slightly affected by the light, a catalytic action sets in, and ex- 

 tends itself to the salts of iron. 



Enlarging of Images. The megascope, invented in 1780, 

 is used to produce large from small proofs ; thus, to obtain from 

 a microscopic negative on glass, a portrait of the natural, size. 

 It never gives an agreeable picture, but skilful retouching may 

 diminish its imperfections. The solar microscope answers well for 

 the same purpose ; the negative being placed in the focus of the 

 objective, and the sensitized paper on a screen in a darkened 

 room. The electric light may be used, but solar is preferable. 



Heligraphy. This is understood to comprise the effects pro- 

 duced by light on non-metallic substances ; but is applied es- 

 pecially to a development of the discoveries of Nicephorus 

 Niepce, which has become very important, since it affords a 

 means of obtaining impressions from metallic plates and litho- 

 graphic stones. Niepce used asphaltum, but Daguerre re- 

 marked that all bituminous resins and the residues of essential 

 oils are decomposed by sunshine. Vegetable juices, also, are 

 sensibly affected by it. In the process employed by M.M. 

 Lemaitre and Niepce de Saint Victor, a carefully cleaned plate 

 of polished steel is coated with a solution of bitumen of Judea 

 in essence of lavender, and dried by heat. A transparent posi- 

 tive is then placed over it, and after the bitumen, which has 

 been rendered soluble by sufficient exposure to light, has been 

 dissolved off by a mixture of rectified oil of naphtha and ben- 

 zinc, it is washed and dried. The plate is next acted on by 

 nitric acid diluted with water and mixed with alcohol, and, 

 having been again washed and dried, it is covered with finely 

 powdered resin, and heated. This hardens the bitumen, and 

 in iho shadows forms granulations which givo good impressions 

 with ink. 



If a picture is obtained with bitumen, by the method of 

 Niepce, and the plate is then placed in an electrotype apparatus, 

 eoppcr will be deposited upon it, on connecting it with the 

 negative polo ; but it will bo corroded in the lights, on con- 

 necting it with the positive. A plate may, thereforo, be obtained 

 which will give impressions like an engraved copper-plate, or 

 like an engraving on wood. 



