74 INFRA-RED REFLECTION SPECTRA. 



back and forth in a horizontal direction in a very accurately milled slot. 

 Two openings, each having an area of 2 by 3 cm., were cut into the sheet 

 of metal, over which and facing the spectrometer slit were attached the 

 mirrors. The silver mirror was, of course, permanently attached over 

 one of the openings in the carrier, and it was a simple matter to attach 

 the previously prepared reflecting surfaces of the minerals over the 

 other opening. This brought the reflecting surfaces of the silver mirror 

 and the mineral into the same plane, and limited the exposed area to the 

 size of the opening in the carrier. The silver mirror was chemically 

 deposited upon a plane glass plate, and was new. 



Occasionally the reflecting surface of the mineral was simply a cleav- 

 age plane for example, selenite and celestite. 



The angle of incidence 

 upon the mirrors was 25. 

 Since we are finding the 

 ratio of the reflecting power 

 of the mineral to that of 



Spectrometer slit 



silver, this large angle, 

 Mineral which was unavoidable on 

 account of the construction 

 of the spectrometer, will not 

 affect the results apprecia- 

 h eater bly. The values given are 



FIG. 59. Arrangement of apparatus for reflection work, sliglltlv higher than the ab- 

 Spectrometer iiot shown. n 



solute reflecting power, but 



the correction is only about 2 per cent 1 at the maximum, and since so 

 much depends upon the condition of the reflecting surface of the min- 

 eral, the correction has not been made, except for the metals. 



The spectrometer slits were 0.3 mm., or about 2" of arc, as in the 

 preceding work. The radiation from the Nernst "heater" was thrown 

 upon the reflecting mirrors by means of a mirror having a focal length 

 of 15 cm. and an aperture of 12 cm. 



As mentioned elsewhere, in a majority of minerals the general absorp- 

 tion increases so rapidly beyond 5 ju, that unless one can reduce the thick- 

 ness to o.oi mm. it is impracticable to examine them. It is not a diffi- 

 cult matter to reduce liquid films to this thickness. On the other hand, 

 to grind a solid to even o.i mm., and then mount it so that it will be free 

 from glue and free from its support, is not practicable in many cases. 

 The reflection method is not much more advantageous, because larger 



1 Hagen and Rubens: Ann. der Phys. (4), ir, 873, 1903, have found the reflect- 

 ing power of silver to be about 98.5 per cent throughout the spectrum beyond 2ft, 



