1889.] Photographs of the red end of the Spectrum. 157 



The cyanin and quinine solution, however, is troublesome to prepare 

 and neither it nor the plates keep well ; moreover the dye is expensive, so 

 that some more simple and more certain stain is desirable, and if alizarin 

 blue, which chemically is allied to cyanin, can take its place for such ob- 

 servations it will be an advantage. I may mention that it has already 

 been tried by Schiendl and Eder, but they do not appear to have re- 

 cognised its gi'eat sensitiveness for the red rays, which is not so apparent 

 with prism specti'oscopes as with gratings and requires the use of yellow 

 or red screens to produce its full effect. 



Dr. Eder has recommended ccerulein as a sensitiser for the red rays, 

 but though it certainly is useful, the maximum of effect is produced be- 

 tween D and B, while with alizarin blue it is between C and A. I have 

 also found it uncertain in its action, but hope to make further trials of 

 it with the gratings, as well as of a dye called gallocyanin, obtained by 

 the action of nitroso-dimethyl-aniline on tannic acid, which also I have 

 found is a strong sensitiser for the red rays. 



I have also lately been trying a new fluorescent phthalein dye, 

 recently introduced by the Badische Anilin and Soda Fabrik, called 

 Rhodamine (the phthalein of meta-amido phenol) which I thought 

 might be likely to be useful in orthochromatic photography for copying 

 coloured drawings &c. It undoubtedly has capabilities in this direction, 

 but so far as I have tried it, seems inferior to erythrosin in sensitiveness 

 for yellow. It is prepared by fusing phthalic anhydride with diethyl- 

 meta-amido-phenol and forms a reddish violet powder, very easily solu- 

 ble in water, the solution being of a bright crimson with a strong orange 

 fluorescence. It is not very soluble in alcohol. Acetic acid dissolves it 

 and the solution shows a very vivid scarlet fluorescence. Ammonia 

 produces no visible change. Dilate mineral acids slightly redden, but do 

 not, as with the eosins, destroy the fluorescence of its watery solutions ; 

 on the contrary they rather heighten it. Nitrate of silver precipitates 

 it from the watery solution, but not readily ; the brick-red precipitate is 

 very soluble in pure water. 



Examined in the spectroscope a sti'ong watery solution absorbs the 

 spectrum strongly in the blue, green and yellow from a | D to F | G, 

 though the only distinct absorption band with a weak solution is about 

 midway between D and E. 



Gelatine dry plates stained with a solution of rhodamine in dis- 

 tilled water at 1 : 10,000 with l°/ of solution of ammonia, show two 

 very marked regions of increased action — one between G and F, the 

 other from E to D, strongest from D to D \ E. The photographs taken 

 with the small spectrograph show a curious but well marked series of 

 bands of increased sensitiveness in the parts of the spectrum where the 



