THE EXTREME INFRA-RED RADIATIONS. 



163 



substances have narrow bands of metallic reflection at various points 

 in the spectrum, and they cau be utilized in the same way to produce 

 pure pencils of rays. With quartz, the principal of these bands are 

 situated at wave lengths of 8// and 21//, while, according to Rubens and 

 Aschkinass, the rays reflected most completely by rock salt and by 

 sylvine have wave lengths respectively of 51// and 61//. This last wave 

 length is exactly one hundred times as great as that of the orange in 

 the visible spectrum. 



The diagram (flg. 1) represents the disposition of apparatus in the 

 investigation of Eubens and Aschkinass. At L is the lamp sending 

 out its radiations to the mirror M t by which the beam is reflected to 

 M 2 . Between these mirrors is placed the grating E. After leaving 

 M 2 the beam suffers five reflections at the surfaces S, proceeds then to 

 the mirror M 3 , and finally falls upon the receiving surface placed in 



Fig. 1. — Diagram showing arrangement of apparatus for studying infra-red spectrum. 



the small reflecting chamber P. By this latter device the rays not 

 absorbed at first contact with the receiving surface are reflected back 

 upon it by the walls of the chamber, and thus serve more effectively to 

 warm it. A series of screens is so arranged as to exclude from this 

 chamber all stray light. 



However interesting we may consider the fact of the existence of 

 these bands of selective reflection, their chief value lies in their appli- 

 cation to the purposes of investigation. No procedure known prior to 

 their discovery could be used to separate homogeneous pencils of rays 

 of these great wave lengths in sufficient intensity for an examination 

 of their properties. 



One of the first studies which have been made with regard to these 

 rays is the transparency of various substances to them. It has been 

 shown that the rays separated by the use of quartz traverse rock salt 



