RESIDUAL RAYS. 



29 



RESIDUAL RAYS 1 FROM MICA. 

 Muscovite Mica [H 2 KAl(SiO\i):i]. 



In fig. 20 arc plotted the spectrometer arm rotations (abscissae) and the 

 galvanometer deflections in the grating spectrum of mica. The sharp 

 maximum at A is the central image, while B x is the selective reflection band 

 (first-order spectrum) in the region of 9 /. For this part of the curve 

 three reflection surfaces were used, while to obtain the part C u C 2 four 

 reflecting surfaces were employed. The latter bands are plotted to a 

 larger scale in C\ and C' v The maxima in the spectrum to the left (not 



1 8 3 4 5 6 



Fig. 20. Muscovite mica. 



plotted) were identical with the above. The wave-lengths of the maxima 

 at C\, C 2 are 18.4 and 21.25 t 1 (Rubens and Nichols) and are used in the 

 calibration curve of grating No. 2, curve b, fig. 19. 



RESIDUAL RAYS FROM CARBONATES. 



Calcite (CaCOs). 



(Grating No. 2. K = 0.2120 mm. 



Fig. 21.) 



The plane cleavage faces of 3 large crystals were used in this exami- 

 nation. The spectrometer slits were 4 mm. wide. In fig. 21 the central 



1 It is a pleasure to note the general adoption of the expression " residual rays " for 

 the meaningless " reststrahlen," formerly used. The English vocabulary seems sufficiently 

 complete to enable writers to find equivalents to " etalon," " entladungsstrahlen," etc, 



