1876.] J. Waterhouse — Use of Eosin in PliotograpMng the Spectrum. 15 



ble to any colour,* experiments must be tried with various dyes before a 

 conclusive decision can be come to. The results of such an examination I 

 hope to lay before the Society on a future occasion. 



A further peculiarity of this d.jQ is that ordinary wet collodion plates 

 prepared with bromo-iodised collodion containing it, exhibit a marked pro- 

 longation of the photographic action of the spectrum in the green and yel- 

 low, extending it beyond its usual limit of h, or at most E, nearly to D. 

 The stained bromo-iodised collodion is strongly fluorescent and i-etains its 

 rosy tinge. Examined with the spectroscope it shows two strong absorp- 

 tion bands in the green. (Fig. 6.) 



The character of the photographic image of the spectrum, as obtained 

 on the stained wet bromo-iodide plates, is entirely different from what it 

 was in the dry bromide plates, and we have an image of fair density showing 

 strong action extending from above Ha to a little below Gr, where there is 

 an abrupt and distinctly marked band of lessened action extending to about 

 half way between F and E, from which point the action decreases to its 

 minimum between 5 and E, and again rises at E with a marked increase of 

 action extending half way to D, whence it gradually decreases till it disap- 

 pears about D. (Fig. 7.) The increase in the extent of the photographic 

 action towards D will be seen by comparing Figs. 7 and 8, the latter of 

 which shows the spectrum as taken upon an unstained wet bromo-iodide 

 plate. 



It is noticeable that a band of decreased action is observed almost 

 corresponding with the position of the space between the absorption bands 

 of the dye, and further investigation may possibly show similar effects with 

 other dyes. 



From this marked sensibility to the green and yellow rays of the spec- 

 trum, it might have been anticipated that wet plates prepared with the eosin- 

 stained collodion would have shown an increased sensitiveness for foliage 

 and other coloured objects of a green or yellow tint, and might have proved 

 of use in photographing coloured maps, paintings or other documents such 

 as the Sanskrit MSS. written on yellow paper. On trying a landscape I 

 found that the dye lessened the sensitiveness of the plate very considerably, 

 and that the exposure had to be increased to about three times what was 

 necessary for similar plates unstained. Even with this increase of exposure, 

 there was little or no improvement in the detail of the foliage, but the image 

 was much denser than usual and the shadows were particularly clean and 

 ■well defined. I also tried photographing bouquets of flowers and a stained 

 glass window comprising red, green, yellow and blue, both with dry bromide 

 and wet bromoiodide plates, but found that little practical advantage was to 



* See paper in Fogg. Annal. Vol. CI. p. -153, translated in Pliil. Mag. S. 4, Vol. 

 47, p. 273. 



