1876.] J. Waterhouse — TTse of Eosm in FhotograpMng the Spectriion. 13 



This effect is quite in accord with Dr. Vogel's theory, that the sensibility of 

 dry collodio-bromide of silver films for any particular part of the spectrum 

 may be heightened by staining them with a suitable dye which absorbs that 

 part but not others ; but so distinct a change of position of the maximum of 

 action from the indigo to the green has not, so far as I am aware, been 

 observed before on films of bromide of silver, though Dr. Vogel has noticed 

 it on films of chloride of silver stained with roseine. [Ber. Deut. Chem. 

 Ges. 1874, p. 516.] 



The dye to which the name of Eosin has been given, from''Ews, the red 

 of the morning dawn, is, according to Hofmann,* the pthalein of dibromre- 

 sorcin, or tetrabromofluorescin, and is soluble both in water and alcohol, the 

 solution being of a bright rosy-orange colour with a strong greenish-yellow 

 fluorescence, tending to green in the watery solution and to yellow in the 

 alcfbholie. Examined with the spectroscope, a weak watery solution shows 

 a strong obscuration of the spectrum from below E to above F, with a 

 strongly marked absorption band about E and h, and a second fainter band 

 about and above F [Plate I, Fig. 2]. A weak alcoholic solution shows 

 similar bands, but displaced more towards the red, the wide band beginning 

 at h and extending to about one-third the distance between E and T), while 

 the fainter band is below F (Fig. 3). 



Dr. Vogel has laid it down as one of the conditions of success in such 

 observations, that the dye employed shall combine chemically with free iodine 

 or bromine, and I was led to specially select this dye for experiment from 

 an anticipation that it might prove particularly suitable for the purpose on 

 account of its being a compound of resorcin, a substance which readily com- 

 bines with bromine and particularly with iodine. 



The dry bromide plates experimented on were prepared in two ways — 

 Xst. — By using bromised collodion coloured with the dye. This collodion 

 showed no fluorescence and was of a bright golden colour inclining to orange, 

 without any ti'ace of the beautiful rosy tint peculiar to the dye. This, 

 however, was probably caused by acidity of the collodion, induced by long 

 keeping, as a more neutral and fresher sample shows a fine yellow fluores- 

 cence and rosy tint. Examined in the spectroscope the absorption bands 

 were absent, or so faint as not to be distinguishable, an effect which is 

 observed with an acid watery solution of the dye. The films given by 

 this collodion were rather transparent and showed only a slight yellowish 

 opalescence by direct transmitted light, but by reflected light, or laid on 

 white paper they showed a distinct pink tint. Examined in the spectroscope, 

 the peculiar absorption bands in the green were not perceptible. 



2nd. — By applying a watery solution of the dye to plates prepared with 

 unstained bromised collodion after the free nitrate of silver had been removed 



* Bcr. Deut. Chem. Ges. VIII. 62, 146, quoted in Am. Jour. Arts, So. May, 1875. 



