1891.] On Sensitisers for Rays of Low Ref ranglbility. 345 



capacity of glass revealed by Professor J. J. Thomson's experiments,* 

 this is only what might be expected. 



,, 



" On the Bisulphite Compounds of Alizarin- blue and 

 Coerulin as Sensitisers for Rays of Low Refrangibility." 

 By GEORGE HIGGS. Communicated by LORD RAYLEIGH, 

 Sec. R.S. Received February 19, 1891. 



The determination of the relative wave-lengths of the Fraunhofer 

 lines, by photographing all the orders of spectra given by any 

 particular grating, includes certain subjects which present more or 

 less difficulty, and that of selecting or producing a dye-bath adapted 

 to the requirements of the two or more orders comprising the subject 

 is intimately connected with that of the choice of absorbing media. 



Having been engaged for some time in investigations of this nature, 

 I had occasion, during the summer of 1889, to require an impression 

 of the 2nd order, about X 3300, contiguous with that of the red end 

 of the 1st order, and finding that the ordinate of an actinic curve for 

 a plate immersed in a very dilute alcoholic ammoniacal solution of 

 cyanin (1 : 30,000), reduced to about one-fourth of that for an unpre- 

 pared plate, I abandoned its use for this purpose. The results 

 appeared to be unaffected by the addition of quinine. 



Subsequently, induline, ccerulin, alizarin-blue, and the bisulphite 

 compounds of the two latter were used. When obtained in a state 

 of sufficient purity the alizarin-blue S leaves little or nothing to be 

 desired, for, whilst possessing, in a high degree, sensitising properties 

 for rays throughout the region comprised between X 6200 and 8000, 

 it does not, like cyanin, lower the sensitiveness to the violet and 

 ultra-violet. 



The following is one of the processes I employed in the prepara- 

 tion of the dye-stuff in a pure state : 



To a saturated solution of sodium bisulphite in a mortar is added 

 alizarin-blue paste. This is disintegrated with a pestle, and poured 

 into a glass vessel capable of holding an additional quantity of sodium 

 bisulphite, in all 10 parts of the paste to 20 parts of bisulphite, and 

 another 10 parts of water. The vessel is well stoppered, set aside in 

 a cool place for five or six weeks, and shaken daily, but left undis- 



bed during the last eight or ten days, 

 he solution is decanted, filtered, and treated with alcohol, to pre- 



itate the greater portion of the remaining sodium bisulphite. 

 50 pavts of water are now added with a sufficiency of sodium chloride 

 >rm a concentrated solution. Again set aside in an open-inouthed 



* ' Roj. Soc. Proc.,' vol. 46. 



2 A 2 



