266 Mr. R. Hunt on the Use of Hydriodic Salts 



67. It is essential to a right understanding of the following 

 results, that the absorptive power of the media I used in my 

 experiments be set forth, Into a frame were fixed four coloured 

 glasses. 



A purple glass, cutting off all the rays below the green, a 

 portion of the green being also absorbed. 



A green glass, admitting the permeation of those rays 

 only which lie between the least refracted extremity of the 

 blue and the extreme orange. A portion of the yellow rays 

 are absorbed. 



A pale amber glass, shortening the spectrum by the vio- 

 let and indigo rays only. 



A red glass, absorbing all the most refrangible rays, per- 

 mitting those only which lie below the blue to permeate. 



68. If any of the sensitive darkened photographic papers 

 («, b 9 c 9 d, o,), washed with a good hydriodic solution, be placed 

 in close contact, face to face, with an engraving which has 

 been rendered transparent by being well soaked in water, and 

 exposed to sunshine with the above frame superposed, we 

 produce (to use Sir J. Herschel's nomenclature) a positive 

 and a negative photograph on the same sheet. Beneath the 

 blue glass the picture is copied as perfectly, but not quite so 

 quickly, as under a colourless glass, the lights of the engraving 

 being correctly copied on the photograph. 



Beneath the green glass the lights and shadows of the pho- 

 tograph are completely reversed. In all the parts which cor- 

 respond with the lights of the engraving the oxidation is much 

 exalted, the paper having assumed a defined blackness. The 

 darker parts of the engraving are copied in lights not simply 

 formed by the contrast of the original brown of the paper 

 with the induced blackness, but by a positive brightening of 

 the parts. 



Beneath the yellow glass the results are singularly uncer- 

 tain. Often on the same sheet, with the same hydriodic so- 

 lution, two experiments will give totally different results. I 

 send you two specimens in proof of this, prepared in every 

 way alike, and both executed within the same half-hour. 



Under the red glass a reversed picture is formed, not by the 

 darkening of the oxide, which retains its original colour, but 

 by the eating out of strong lights under the dark parts of the 

 engraving. 



69. From these results it is evident that the blackening 

 action on the wet hydriodidated paper is dependent on a 

 different class of rays from those which blacken the finished 

 drawings : in the wet process I ever find the maximum of 

 darkness beneath the green glass, or rather within the limits 

 of the green and yellow rays, little or no darkening effect 

 being evident in the red rays; whereas on the dry picture 



