166 



Capt. Abney. On the Effect of the [Dec. 15, 



prepared as emulsions ; when in paper they were prepared by soaking 

 it in a soluble haloid salt, and floating on solution of silver nitrate. 

 The question of the production of sensitive silver haloid salts on a 

 metallic silver plate I have left to be considered later, since it has no 

 direct bearing on the points I wish to discuss in this communication. 



Exposures. — When it was desired to obtain the expression of the 

 action of the spectrum by its direct effect without the intermediary 

 of a developer, the slit of the spectroscope was opened to a width of 

 ^ of an inch, and the exposure prolonged for five to twenty minutes. 

 When the effects had to be shown by development the slit was closed 

 to of an inch, and exposure given varying between J second and 

 one minute or even two minutes. By having a shutter at the slit of the 

 spectroscope it was easy to give two exposures on the same plate or 

 paper, using half the length of the slit for each exposure. This was 

 excessively convenient, since it allowed the different phenomena arising 

 from different methods of exposure to be accurately compared together. 

 The principle on which the exposures were given was as follows : — 

 1st. An exposure was given to the plate, when a pale solution of chro- 

 niate of potash so dilute as to cut off the spectrum above E, was placed 

 in front of the slit. This exposure was in all cases prolonged in order 

 to see if there was any action produced, however feeble, by the spectrum 

 remaining unabsorbed. The next exposure was always taken with the 

 slit unshaded, and on the same plate (or paper) as the first exposure. 

 After a certain interval of time had elapsed, the yellow chromate 

 was again placed in front of the slit, and the exposure continued. 

 The reason for adopting this plan was that the effect of diffused white 

 light (diffused from the prisms during unshaded exposure) would 

 thus be differentiated. Thus, supposing it was found that the first 

 exposure caused no sign of a change in the sensitive salt by the 

 exposure to the spectrum unabsorbed by the chromate, but that the 

 unshaded spectrum caused an action on these parts, it would be evident 

 that the action of diffused light had played a part in causing such an 

 action. 



When such phenomena resulted, plates or papers were first exposed 

 to the unshaded spectrum through the chromate solution, then with- 

 drawn from the camera and exposed to the diffused light of the 

 laboratory for a fraction of a second, or for eight or ten seconds, accord- 

 ing as the experiment was to be conducted by development or by 

 direct printing action, and again inserted in the slide and exposed to 

 the action of the partially absorbed spectrum. If the experiments 

 were rightly conducted, the results of the last two should be con- 

 firmatory of the first two exposures. Other plates or papers were then 

 exposed, giving, unshaded, one half of the slit of a short period, and 

 the other half for a period ten to twenty times as long. By this system 

 nil the phenomena met with could be differentiated and traced. 



