[mclennan-dearle] spectrum OF THE MERCURY ARC 



183 



Company. The radiation from the lamp was allowed to fall upon a 

 large concave mirror having a diameter of 19 cms and a focal length of 

 30 cms which brought it to a focus on the slit Si. From this slit the 



'0 <iALVAnoHil.TlZ 



Fig. 3. 



rays passed to the nickel-steel concave mirror Mi thence through the 

 rock salt prism P to the plane nickel steel mirror M 2. From this they 

 were reflected to the concave nickel-steel mJrror M 3 and by it they 

 were brought to a focus on the linear junctions of the thermopile at 

 T, which was placed immediately behind the slit S2. The prism and 

 plane mirror were mounted on a table which rotated about the point 

 A. By turning this table through a small angle, any desired part of 

 the spectrum could be brought to a focus at S2. The rotation was 

 produced by the motion of a helical drum attached at D, which was 

 calibrated in wave-lengths up to lO^u from data on the dispersion of 

 rock salt as given by Paschen^ and others. 



An eye-piece E was attachable behind the slit S2 for the purpose of 

 focussing lines in the visible part of the spectrum on the thermopile 

 and of adjusting the prism so that the radiation brought to a focus 

 at S3 was in agreement with the reading on the drum. The prism 

 had faces 3 • 2 cms by 4 • 2 cms and was ground to an angle of approxi- 

 mately 55°. Judging by the visible spectrum, there was very little 

 curvature in the spectral lines produced by this prism. 



iPaschen, Ann. d. Phys. 26. 1. pp. 120-138; 5 pp. 1029-1030, 1908. 



