CHAPTER I. 



INTRODUCTION. 



With the accumulation of data the evidence becomes more and more 

 convincing that there are no strong bands of selective absorption (except 

 in the colored glasses given on a later page) in the region of short wave- 

 lengths, near the visible spectrum, such as are to be found beyond 6 /*. 

 Material suitable for investigation is obtained with difficulty. The present 

 contributions to the subject of transmission spectra is rather meager in 

 volume, but several of the substances show peculiarities worth recording. 



The apparatus and methods of examining the following material, con- 

 sisting of a rock-salt prism and mirror spectrometer, were described in 

 Carnegie Publication No. 65. Instead of the radiometer, an improved 

 Rubens 1 thermopile was used to measure the energy in the transmission 

 spectra. 



GROUP I: TRANSMISSION SPECTRA OF VARIOUS SOLIDS. 



Molybdenite (MoS). 



(From Yorkes Peninsula, South Australia. Foliated, distorted; thickness, J =0.05 and 0.31 



mm.; fig. 25.) 



In the preceding work it was noticed that the sulphides are, in general, 

 quite transparent. Sphalerite (ZnS) was found to have wide absorption 



5 6 7 8 9 



Fig. 25. Molybdenite. 



10 



12 



13 



I4JU- 



bands at 3 and 15 fx. On the other hand, stibnite (Sb 2 S 3 ) is opaque to the 

 visible and has a second absorption band beyond 1 5 <x. In the interven- 



1 See Appendix II. 



4i 



