OF ARTS AND SCIENCES. 219 



If an aqueous solution of eosin (fluorescein, a rather complicated 

 organic compound) be added to a neutral solution of silver nitrate, a 

 colored precipitate is thrown down. It seems to me that the precipi- 

 tate obtained from the addition of silver nitrate to the eosin is a defi- 

 nite salt of silver ; and this is shown in the following way : "Wash the 

 precipitate of eosin and silver nitrate with distilled vi^ater, until all 

 trace of an excess of silver nitrate is removed ; then dissolve this 

 washed precipitate in a strong solution of sodium hyposulphite, until 

 the latter is saturated ; then add to this saturated solution a solution 

 of cadmium or potassium bromide. A preci[)itate is thrown down, 

 which is again soluble in more of the sodium hyposulphite. Now, if 

 this salt of silver eosin be precipitated upon a film which contains 

 neither bromide, chloride, or iodide of silver, a definite picture of the 

 green rays of the spectrum will be obtained. This image which I 

 now exhibit corresponds to that part of the spectrum which this silver 

 salt absorbs, as may be seen on comparing it with the absorption 

 spectrum of an emulsion of this salt which has been prepared for 

 that purpose. 



Commercial aniline chloride is a salt of which the coloring matter 

 in solution absorbs violet, and also the green rays between F and D 

 lines of the solar spectrum; its greatest intensity being at E line of 

 the solar spectrum. Here is a definite image of those rays, which 

 corresponds with absorption-bands of this silver salt. All three plates 

 were exposed to the spectrum for about ten minutes only. We will 

 go still further. Silver iodide absorbs the violet and the more re- 

 frangible blue rays. It is these rays only that reduce this silver salt. 

 In other words, 2 (Ag. I.) is reduced to Ag? I., on its exposure to the 

 rays of light which this salt absorbs. Argentic chloride, on exposure 

 to those refrangible blue rays which it absorbs, becomes argentous 

 chloride. Argentic bromide, on exposure to those refrangible blue 

 rays (down to line F ) which it absorbs, becomes argentous bi'o- 

 mide. The intensity of photographical action corresponds to that of 

 absorption. Neither of these salts are affected by rays of less refran- 

 gibility than those which they absorb ; yet there may be slight action 

 extending below the invisible band, corresponding with an extended 

 and faint absorption below this point. I would therefore deduce a 

 general proposition founded on these experiments, and expressed in the 

 following terms : — 



A colored silver salt is reduced by rays of light of the same refrangi- 

 hility which it absorbs. 



I would propose the following theory as being very probable : The 



