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 Avell 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 iifterwards 

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

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

 the hyposulphite, remains in the paper is great, and it will tend much 

 to preserve these beautiful transcrij^ts of nature. 



The i^atentee also claims as an improvement on the use of var- 

 nished or other trans2:)arent paper 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 with 

 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 folloAving the |)rocess 

 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 couA'enience of using paper 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 views of scenery which 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 straiglitened; but the case is met by employing talc, varnished 

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

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

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

 The next improvement consists in forming photographic pictures 

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

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

 is mixed with 200 parts of albumen, and spread as evenly as possible 

 upon the surface of a steel plate and dried by the heat of a gentle 

 fire. The i)late is then taken, and, while still warm, is washed over 

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



