238 M. E. Pringsheim on a Measurement of Wave-lengths 

 the figure ; in reality the rays were caused to pass as near 



Fig. 1. 



one another as the breadth of the strips forming the slit s 

 permitted. 



The torsion-apparatus, in order to protect it from all lateral 

 radiations and from air-currents, was covered with a tin case 

 filled with wadding, in which was an aperture opposite the 

 mica plate to admit the rays. In front of this aperture a 

 pasteboard tube P 2 , passing through a vertical pasteboard 

 disk, was fixed. Opposite to the mirror of the torsion- 

 apparatus, at a distance of about 1040 millim., was the milli- 

 metre-scale M, upon which the lamp L, with the aid of the 

 lens I and a second aperture in the case K, produced the 

 light-reflection I/. 



As the action of the rays upon our apparatus extended far 

 beyond the dark interval between the spectra of the first and 

 second orders, and consequently the action of the luminous 

 rays of the second spectrum combined with that of the heat- 

 rays of the first, these two locally coincident spectra had to be 

 separated. For this purpose, at first a solution of iodine in 

 sulphide of carbon was used, which was inserted at J in the 

 path of the rays, and absorbed all the visible ones, while it 

 transmitted the greatest part of the heat-rays. The solution 

 was contained in a glass tube having its two extremities 



