'!68 Prof. Draper on the Interference Spectrum, 



inch being divided into one hundred millions of equal 

 parts : — 



Length of wave corresponding to the fixed line B 254-1 parts 



C 2422 ... 



D2175 ... 



E 194-5 ... 



F 1794 ... 



... ... ... *.. \j 1587 ... 



H 1464 ... 



When, therefore, we have any tithonic, photic, thermic, 

 phosphorogenic, or any other actinic effect under discussion, 

 in relation to the spectrum, we have only to determine its 

 place among the fixed lines, remembering in the interference 

 spectrum the simple law that connects the deviations and wave- 

 lengths. In the mode of operating here described, absokite 

 exactitude is not reached, because our measures are obtained 

 from a flat surface and not upon a circular arc. 



To the interference spectrum thus formed, I exposed for 

 half an hour a Daguerreotype plate, rendered sensitive by 

 iodine and then by bromine. It resulted that the bromide of 

 silver is decomposed at a maximum by a wave of tithonicity, 

 which is 0'00001538 of a Paris inch in length. The action 

 does not extend equally, as we might have supposed, towards 

 the more and less refrannrible regions. 



I exposed a silver plate which had been prepared by iodine, 

 bromine and chloride of iodine successively. The point of 

 maximum fell, as before, at 0*00001538. 'i'he time of expo- 

 sure one hour. The decomposition commenced by a wave in 

 the green space, the length of which was 0*00002007, and was 

 terminated by one in the violet, whose length was 0*00001257. 

 The point of maximum action, therefore, inclined to the violet, 

 and was not midway between the extremities of the tithono- 

 graph. The absolute length of the stain depends however on 

 the time of exposure. 



I need not multiply these results. It is sufficient to add, 

 that in several trials I obtained, in these apparently delicate 

 experiments, tithonographs of the interference spectrum, on 

 different surfaces, in great perfection. The fixed lines, which 

 are crowded close together, were beautifully distinct. 



I would suggest, therefore, that when we wish to indicate 

 spectrum regions with precision, we should use wave-lengths. 

 By doing this we shall connect the various actinic phsenomena, 

 — whether they refer to light, heat, tithonic, or phosphorogenic 

 rays, — with a great many of the numerical results of optics, 

 and have fixed points of comparison. 



