RESIDUAL RAYS. 



2 7 



contained "ghosts." For the main part of the investigation grating No. 2 

 was used, which on account of the finer wires was not so regular in its 

 winding. Using three reflections from quartz it was possible to observe 

 the third order maximum of the band of wave-length, 9.05 ll; in all other 

 cases, the third order spectrum was too weak for observation. A grating 

 having wires of more uniform diameter and more uniformly wound, e.g., 

 wound on a screw thread, would no doubt have been more efficient in 

 producing a pure spectrum. 





] ^2 





N 



Fig. 18. 



The spectrometer arm carrying the Nernst "heater," the slit S t and 

 the mirror, m u was movable. The plane of the grating was made normal 

 to the beam of light by placing a mirror on the wires and revolving the 

 grating until an image of the slit, S 1} was projected back upon the slit. 

 It was not convenient to have the collimating mirror and the grating revolve 

 about the spectrometer axis, which is necessary to maintain the same grat- 

 ing constant. Accordingly the grating was kept in a fixed position and 

 the spectrometer arm, carrying the mirror and source, was revolved 

 about it. 



The apparatus was calibrated by locating the maxima of the selective 

 reflection bands of substances previously examined by Rubens and Nichols 

 and by Aschkinass, viz, quartz at 8.7 and 20.75 <"> m i ca at J 8-4 an d 21.25 /*> 

 fluorite at 24.4 /x, and calcite at 29.4 \x. The calibration curves are shown 

 in fig. 19, where curve a is for the grating having a constant -KT= 0.3279 mm. 

 (for Na), and curve b is for the grating (No. 2) having a constant 2T=o.2i20 

 mm. (for Na). In this curve the abscissae are the maxima of reflection 

 bands and the ordinates are the rotations of the spectrometer arm from 

 the zero position. Blocks of wood with vertical metal plates, having 

 openings 2 by 3 to 4 by 5 cm. were mounted securely upon a board, as 

 shown at r v r 2 , r S) r 4 . The minerals were secured to the back of the metal 



