46 INFRA-RED TRANSMISSION SPECTRA. 



Fluokite (CaF). 



(Curve c, / = 2.s8 mm.; curve d, t = io mm.; curve e, / = i.84 mm.; curve/, /=3-85 mm. 



light-green color; fig. 29.) 



On account of the increased scarcity of this material, it is of interest 

 to determine the effect of inclusions upon the transmission. In fig. 29, 

 curve c was a perfectly clear specimen; while curve e contained numerous 

 small inclusions, or, perhaps more exactly, small cleavage planes. The 

 latter when held at a distance of 30 to 50 cm. from the eye appeared quite 

 blurred. Nevertheless, throughout the infra-red the loss of energy is only 

 about 2 per cent greater than that of clear fluorite. This is to be expected; 

 for each cleavage plane reflects some of the light, and the magnitude of 

 this reflection, since it depends upon the refractive index, decreases with 

 increase in wave-length. Curve / shows the transmission of a specimen 

 of light yellowish-green fluorite (see Carnegie Publication, No. 65, p. 69, 

 for another example) which has an absorption band at 1.4 /<, hence not 

 suitable for a prism. This specimen was free from inclusions. 



Carbon (C). 

 (Curves a, b, c, lampblack; curve d, diamond; fig. 30.) 



The commonest and most conspicuous example of the effect of structure 

 upon absorption is to be found in carbon in the form of lampblack and 



100% 



t=. 009mm 



3 4 5 6 7 



Fig. 30. Carbon (a), (6), (c); Diamond. 



of diamond. Since the observations on these two substances are published 

 in separate and rather isolated places they are incorporated here for com- 

 pleteness of illustration of the effect of structure. The transmission curves 



