the Developed Photographic Image, 205 



that the phenomena are absent, a reversal being almost impos- 

 sible to obtain unless the length of exposure be such as to 

 thoroughly oxidize the nitrite at the expense of gelatine. For 

 ordinary purposes it may be said that a reversal is non-existent 

 under these conditions. 



If a plate be exposed in benzene, however (a liquid which 

 does not permeate through gelatine), the phenomena are still 

 existent. If a plate be exposed to such an extent that there 

 is a marked image apparent before development, and be then 

 immersed in water, it will be found that when the image 

 appears the gelatine refuses to swell to the same extent that 

 it does when the light has not acted. Taking these two expe- 

 riments together, it is evident that the gelatine has played 

 some part with the silver bromide. It may therefore be 

 presumed that the three last phenomena are due, the 1st to 

 the oxidation of the surface-particles of the bromide and a 

 consequent change in colour, the 2nd to the change in colour 

 of these particles permitting the coloured rays to which it is 

 sensitive to strike a deeper layer, and the 3rd to the oxidation 

 of this layer at the expense of the gelatine. The 3rd and 4th 

 phenomena are so unimportant that they are scarcely worth 

 investigating. The presence of organic matter is evidently 

 necessary for their appearance ; at least I have never been 

 able to obtain them with collodion films not containing a pre- 

 servative. 



As before, the experiment of saturating one of these gela- 

 tine films with bicromate of potash shows that the reversing 

 action is very much increased by the presence of the oxidizing 

 agent. Mr. Bolas* has recently described a plan of producing 

 reversed negatives by allowing the bichromate to dry in the 

 film, which is a practical application of this reversing action 

 of light in the presence of an oxidizing agent. 



A convenient method of showing these phenomena on the 

 same plate is to use a screen containing squares of gradu- 

 ated opacity, as suggested by the editor of the ' Photographic 

 News,' or such as the sensitometer prepared by Mr. Warnerke, 

 and procurable at most photographic warehouses. 



Having treated of these reversals of the image in a general 

 way, it now remains to show which radiations are effective in 

 producing them. For testing this, spectro-photography was 

 resorted to, a special dark slide having been constructed ca- 

 pable of holding a cell which would contain the plate, and 

 be immersed in a liquid or any gas or vapour whose action it 

 might be desired to test. Three flint-glass prisms were used, 

 and a lens to the camera of about 2 feet equivalent focus, the 

 * Photographic Journal, May 1880. 



