306 THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 



For this purpose the print, after undergoing the usual fixing proc- 

 ess, is dipped into a boiling solution of strong caustic potash, which 

 changes the color of the print, and usually, after a certain time, ac- 

 quires something of a greenish tint, which indicates that the process 

 is terminated. 



The picture is then well washed and dried, and if the tint acquired 

 by it is not pleasing to the eye a slight exposure to the vapors of sul- 

 phureted hydrogen will restore to it an agreeable brown or sepia 

 tint. Under this treatment the picture diminishes in size, insomuch 

 that if it were previously cut in two and one part submitted to the 

 potash process and the other not, the two halves when afterwards 

 put together would be found not to correspond. The advantages of 

 this process for removing any iodine which, even after fixing Avith 

 the hyposulphite, remains in the paj^er is great, and it will tend much 

 to preserve these beautiful transcrij^ts of nature. 



The patentee also claims as an improvement on the use of var- 

 nished or other transparent pa])er impervious to water, as a sub- 

 stitute for glass in certain circumstances, to support a film of albu- 

 men for photographic purposes. A sheet of writing paper is brushed 

 over with several coats of varnish on each side; it thus becomes 

 extremely transparent. It is then brushed over on one side witli 

 albumen, or a mixture of albumen and gelatin and dried. This 

 film of albumen is capable of being rendered sensitive to light by 

 exposing it to the vapour of iodine, and by following the process 

 indicated in the preceding section of this specification. The ad- 

 vantages of using varnished or oil paper do not consist in any su- 

 periority of the images over those obtained upon glass, but in the 

 greater convenience of using pajjer than glass in cases where a large 

 number of pictures have to be made and carried about for consid- 

 erable distances; besides this, there is a well-known kind of photo- 

 graphic pictures giving panoramic Adews of scenery Avhich are pro- 

 duced upon a curved surface by a movement of the object glass 

 of the camera. To the production of these images glass is hardly 

 applicable, since it can not be readily bent to the required curve and 

 again straightened ; but the case is met by employing talc, varnished 

 paper, oiled paper, etc., instead of glass. Tt will be seen that the 

 varnished. paper acts as a support to the film of albumen or gelatin, 

 Avhich is the surface on which the light acts and forms the picture. 

 The next improvement consists in forming photogi'aphic pictures 

 or images on the surfaces of polished steel plates. For this purpose 

 one part (by measuiv) of a standard solution of iodide of potassium 

 is mixed Avith '200 jjarts of albumen, and spread as evenly as possible 

 upon the surface of a steel i)late and dried by the heat of a gentle 

 fire. The plate is then taken, and, while still warm, is Avashed over 

 Avith an alcoholic solution of gallo-nitrate of silver of moderate 



