158 Photographs of the red end of the Spectrum. [April, 



action of the dye is most marked. The action of the yellow rays about 

 D is quite as strong as, if not stronger than on plates stained with the 

 best of the eosin dyes, but for copying coloured pictures I have not 

 found rhodanrine with ammonia equal to erythrosin either with or without 

 a yellow screen ; further trial is, however, requisite. 



The addition of a small quantity of nitrate of silver to the ammo- 

 niacal solution of rhodamine greatly increases the general sensitiveness 

 of the plates, but does not seem to heighten the yellow- sensitiveness, 

 either with the spectrum or with coloured pictures, so much as it does 

 with erythrosin. 



It is, however, noticeable that plates stained with a solution of the 

 dye containing acetic acid in place of ammonia appear almost as sensitive 

 in the yellow as those prepared with ammonia and give about equally 

 good results for copying work : if on further experience this is proved 

 and found to work with certainty, it will be of importance, because 

 the use of eosin dyes in conjunction with ammonia for staining gelatine 

 dry plates for orthochromatic photography has been protected by 

 various patents. It is possible that further experiments with acid- 

 stained plates in conjunction with the use of specially suitable yellow 

 screens might result in a method of using this dye for orthochromatic 

 photography which would be efficient and open to general use. 



In photographing the spectrum with the concave gratings I have 

 found the rhodamine-stained plates useful for the region immediately 

 about D. On a plate I have with me, showing the D lines in the second 

 order, 7 lines can be clearly seen between D x and D 2 , though 11 are 

 sometimes visible to the eye. 



Another new dye from the same manufactory called Nile Blue, 

 which also belongs to the rhodamine series, shows considerable sen- 

 sitiveness to the region between D and B, by the addition of ammonia ; 

 though not so much so as alizarin blue. Without ammonia the general 

 sensitiveness of the plate is much reduced, as Dr. Eder has remarked 

 is the case with most blue dyes, and the image of the spectrum does not 

 extend beyond b. 



It is noticeable that the spectrum taken in the small spectrograph on 

 plates stained with the ammoniacal solution of this dj 7 e shows numerous 

 narrow bands of extra sensitiveness similar to those in the spectrum of 

 rhodamine, showing that both dyes belong to the same group. These 

 bands in the photographed spectra of the two dyes would be an inter- 

 esting subject for further enquiry. 



I have to acknowledge the kindness with which the managers of the 

 Badische Anilin and Soda Fabrik have supplied me with samples of the 

 dyes referred to. 



