42 MEMORIE DELLA SOCIBTa' 



ethereal vibrations, whicb give rise to one or other of these three tbrms of force, 

 depending on the snrface iipon whicli the beam falls. Actinie effects may be pro- 

 duced by any part of the spectrnm , if the receiviiig snrface has the proper che- 

 mical composition, and the maxima of action vary with the photographic compound 

 employed. In collodion, as ordiuarily nsed, the spectrum near requires about 10 

 times as long an exposnre as tho part near G. I bave therefore resorted to dia- 

 phragms to reduce this excess of action, and produce the uniformity bere presented. 



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