1883.J Absorption Spectrum of Iodine in Carbon Disulphide. 481 



disulphide. He, however, solely regarded the visible spectrum from 

 near B to above Gr, about wave-length 4,000. 



Our attention was recently directed to the absorption of this sub- 

 stance, and we have determined photographically the spectra for 

 different thicknesses and strengths of the solution. 



Our inquiries into the behaviour of carbon disulphide had shown 

 us that it, as well as iodine, was transparent for rays of very low 

 refrangibility, as was first shown by Professor Tyndall. We are not 

 aware, however, that any investigations into the absorption at the 

 more refrangible end of the spectrum have been published. An idea 

 seems to be prevalent that the absorption commences in the green, 

 and gradually extends, as the thickness or density of the solution is 

 increased, in both directions until finally all the visible rays are 

 extinguished, leaving the invisible rays at both ends unabsorbed. 



Though one of us employed the solution before to cut off the 

 visible portion of the spectrum (" Phil. Trans.," Part II, 1880, p. 664) 

 in diffraction photography of the lower part of the spectrum, the use 

 of it was abandoned owing to the opinion that was expressed that 

 instead of the infra-red being photographed, the ultra-violet and violet 

 of the spectrum of the next order might be that which was really 

 impressed. Our recent experiments, however, show that no fear need 

 have been entertained on this account, as the annexed diagram will 

 show.* 



The following thicknesses and -strengths of solution were employed, 

 being placed before the slit of the photographic spectroscope : — 



No. I .. 



n n .. 



„ III . . 



„ iv.. 



Iodine in CS 2 . Thickness. 

 . 4 per cent, equivalent to 2 millims. 

 - 8 -jj „, 2 ,, 



• 12 2 „ 



• 16 „ „ 2 „ 



• 32 ,, ,, 2 ,, 



The source of light was usually the crater of the positive pole of the 

 electric lamp, though in two instances we were favoured with a 

 glimpse of sunshine, which enabled us to confirm what we had before 



* This diagram represents the absorption of the continuous spectrum. The 

 bright bands in the electric light above K had power to penetrate the solutions II 

 and III. 



