﻿of Chlorate of Potash Crystals. 



69 



the parallel strata, or possibly to the large amplitudes of the 

 disturbances reflected from the outer surfaces. 



The films, after having been prepared in the manner de- 

 scribed above, were mounted in front of the slit of a small 

 quartz spectrograph, and oriented so as to throw the light 

 from a cadmium spark into the instrument. 



A series of photographs obtained in this way is reproduced 

 in fig. 1 (Plate I.). In some cases a number of bands appear 

 in the ultra-violet, and in other cases only one or two. It is 

 obvious that the thicker the reflecting strata, the larger will be 

 the number of regions selectively reflected in a given spectral 

 range. The approximate values of the lengths of the reflected 

 waves are given in the following table : — 





1st Order Colour. 



Higher Order Colours. 



A... 



1-6 n 



•80 



•54 -40 -30 -26 -23 



B ... 



•617 



•31 





C ... 



•53 



-28 





])... 



If 



•53 



•28 



E... 



64 



•325 



•22 



V ... 



1-26 u 



•633 



•435 -32 -23 



G... 



1-13 



•565 



•39 -235 



H... 



1-23 



•614 



•42 -326 -206 -226 



I ... 



•44 



•23 





J ... 



•60 



•30 





In all except A and J the cadmium-spark spectrum is 

 recorded for comparison. 



It will be noticed that certain films (spectra F and G for 

 example) reflect double bands, which are often as close 

 together as the yellow mercury lines. One film which I 

 examined showed a double line in the red the components of 

 which were just barely resolvable. Its appearance reminded 

 one of the sodium lines when on the point of fusing together 

 owing to the density of the vapour. There seems to be no 

 way of explaining cases of this sort except by assuming a 

 double set of twin planes. In the case above referred to there 

 may have been two thin crystalline plates stuck together. 

 The specimen unfortuately was destroyed so that further 

 examination was impossible. 



A search for the predicted bands in the infra-red region 

 was undertaken at my request by Mr. A. H. Pfund, Fellow 

 in Physics. The light from a Nernst lamp was reflected from 

 one of the purple films, into the slit of a reflecting spectro- 

 meter furnished with a rock-salt prism, and radiometer. This 

 particular film showed a sharp band in the red at wave-length 



