172 PROCEEDINGS OP THE AMERICAN ACADEMY 



on account of a more minute subdivision of the I'educed silver, gives a 

 better-defined and sharp image. 



5. Silver bromide in gelatine (gelatino-bromide emulsion) shows 

 much more action to all rays of the visible spectrum than when this 

 salt is suspended in collodion : the strength of the image gradually 

 diminishes from the blue to the red end, but gives no selective action 

 below line F. 



6. Some of the aniline dyes dissolved in a collodion sensitized with 

 silver bromide containing an excess of silver nitrate and then dried, give 

 in my hands very positive results, which, in detail, are as follows : It is 

 well known that solar light passing through a solution of magenta red, 

 fuchsin, water-blue, or ceosin, all being aniline colors, shows selective 

 absorption of certain rays of the solar spectrum. If a silver bromide 

 collodion be faintly colored with ceosin, for instance, then immersed 

 in a strong bath of silver nitrate, so that there shall be over the sur- 

 face of the collodion an excess of silver nitrate, which after immersion 

 should be thoroughly washed off, and dried in the dark; then if this 

 be exposed to the image of the spectrum in focus, a stronger image 

 of the solar lines will be developed upon the collodion between E and 

 D lines, the maximum of action being between these lines; and a faint 

 image will be developed between the F and E lines. In other words, 

 there is a selective photographic action in that {)ortion of the spectrum 

 (viz., between E and D) where the absorption band of the sunlight 

 colored by this aniline dye is usually observed. Attention is specially 

 called to the fact, that, if the pigment is added to the sensitized collo- 

 dion after the excess of silver nitrate has been washed off, the effects 

 above mentioned do not follow. We have taken advantage of this 

 phenomenon to photograph quite distinct images of solar lines be- 

 tween F and B, which were refracted by two dense glass prisms* and 

 projected by a common 40-in. spectacle lens. I exhibit these pho- 

 tographs to the Academy. I also exhibit the photographic image of 

 the absorption bands of blood, of didymium nitrate and potassic per- 

 manganate in solution ; likewise, a solution of grass chlorophyll. 

 This latter substance offers unusual difficulties, from the fact that, in 

 order to get chlorophyll absorption bands, the sunlight must be very 

 strongly colored with chlorophyll, which circumstance allows an 

 amount of light insufficient to effect the silver reduction, unless under 

 a prolonged exposure. This plate was exposed one and a half hours ; 



* These prisms were made by Adam Hilger of London, being duplicates of 

 those he made for Colonel Abney for his photographic experiments. 



