146 INFRA-RED EMISSION SPECTRA. 



the bands of residual rays reflected from calcite are elliptically polarized. 

 Pfliiger 1 studied the polarized radiation from tourmaline heated to high 

 temperature. Further than this, nothing is known concerning the polari- 

 zation of the radiation emitted by substances like the silicates; hence this 

 test might not be very decisive. 



To subject the above conclusions to experiment in which the reflection 

 of a rough surface is to be measured will be accompanied with difficulties 

 because of the smallness of the surfaces that can be used. 2 In the case 

 of the moon an image of the whole, or a greater part of the surface, may 

 be projected upon the spectrometer slit. This means concentrating radia- 

 tion which comes from a surface many miles in diameter, as compared 

 with a surface which, when produced in the laboratory, amounts to only 

 a few square centimeters. 



1 Pfliiger: Ann. der Phys., 7, p. 800, 1902. 



2 Since writing this, Dr. A. Trowbridge, at the Washington meeting of the American 

 Physical Society, April 24-25, 1908, described a series of experiments on the diffuse reflection 

 of infra-red energy, in which he showed that powdered quartz has minima of diffuse reflec- 

 tion in the regions where there are absorption bands (e. g., 2.95 /*), and reflection maxima 

 at 8 to 9 fi, just as obtains for plane surfaces. This is an excellent illustration of the dif- 

 ference of what corresponds to body color and surface color in the visible spectrum (see 

 Wood's Optics, p. 352). It also illustrates the question of diffuse selective reflection discussed 

 on a previous page. 



