( 346 ) 



p" 



not alter their shape through tension. At the same time the rubber 

 insulates A and B electrically from JSfN'. The four ends of the 

 narrow tubes which stick out are kept cool by mantles with streaming 

 water (these are not represented in the figure). 



If now an electric current is passed through A or B, the tempera- 

 ture of this tube rises a little above that of its surroundings; if an 

 air-current is passed through it, the temperature falls a little below 

 that of its surroundings. The intensities of the currents and, conse- 

 quently, the differences of temperature can in either case be easily 

 regulated and kept constant for a long time. 



Fig 3 gives a sketch of the whole arrangement. 

 The light of the positive carbon L is concentrated by 

 •* the lens E on a screen Q, having a slit-shaped aper- 

 ture of adjustable breadth. The lens F forms in the 

 plane of the slit S of the spectrograph a sharp image 

 of the diaphragm P. The optical axis of the two lenses 

 passes through the middle of the tube containing the 

 sodium vapour, exactly between the two small tubes 

 A and B. 



If now the opening in the diaphragm F has the 

 shape of a vertical narrow slit and if its image falls 

 exactly on the slit of the spectograph, then in this latter 

 the continuous spectrum of the arc-light appears with 

 great brightness. If the tube JVN' is not heated, D^ 

 and 7^3 are seen as extremely fine dark lines, attri- 

 buted to absorption by the sodium, which is always 

 present in the neighbourhood of the carbons. In order 

 that this phenomenon might always be present in the 

 field of view of the spectograph as a comparison 

 spectrum, also when the tube is heated, a small totally 

 reflecting prism was placed before part of the slit S, 

 to which part of the principal beam of light was led 

 by a simple combination of lenses and mirrors without 

 passing the electric furnace. So on each photograph that 

 was taken the unmodified spectrum of the source is 

 also seen. 



The spectral arrangement used consists of a plane 

 diffraction grating 10 cms. diameter (ruled surface 8 

 by 5 cms.) with 14436 lines to the inch, and two sil- 

 vered mirrors of Zeiss; the collimator mirror has a 

 Fig. 3. focal distance of 150 cms., the other of 250 cms. Most 

 of the work was done in the second spectrum. 



