CHAPTER IV. 



REFLECTION SPECTRA IN THE EXTREME INFRA-RED. 



From the foregoing work on selective reflection in which a single re- 

 flecting surface was used, it is apparent that by successive reflection of 

 heat-waves from several surfaces there will remain only the residual rays 

 lying in the region of selective reflection. It was noticed that the reflecting 

 power in the region of 4 to 8 p. is only about -^ that of the band of selec- 

 tive reflection. Hence, after reflecting from three surfaces the intensity 



would be only -j^oo' an< ^ a ^ ter nve reflections only xooVoo' 



Rubens and Nichols x were the first to apply this method in locating 

 the maxima of the residual rays of a series of substances including quartz, 

 mica, fluorite, rock-salt, sylvite, crown and flint glass, sulphur, alum, and 

 calcite. By using a grating of fine wire they were able to extend their 

 observations to 61 //, the longest heat-waves yet identified. Of the above- 

 mentioned substances, only the first four were found to have bands of 

 residual rays in the extreme infra-red. 



Aschkinass 2 did some further work on this subject, examining marble, 

 calcite, selenite, alum, sodium bromide, and potassium bromide. He 

 found a band of residual rays at 29.4 jj. in marble, and showed that similar 

 bands exist in the bromides, the maxima of which lie beyond 60 jj.. 



From the results obtained with the silicates (see Carnegie Publication 

 No. 65, pp. 80 to 90), especially glass, it becomes apparent that one can 

 hardly expect to locate bands of residual rays in the extreme infra-red, even 

 at 18 to 20 /<, unless they are much more intense than those found in the 

 region of 8 to 10^. For it is necessary to have several reflecting surfaces 

 to eliminate the large amount of energy of short wave-lengths as compared 

 with the small amount to be measured, of the long wave-lengths. As is 

 well known now, in the case of quartz, rock-salt, and sylvite, this is an 

 easy matter on account of the high reflecting power of these bands. For 

 example, for the band at 61 n, which was examined after reflection from 

 five surfaces of sylvite, it was found that the reflecting power of sylvite is 

 80 per cent. The galvanometer deflections were only about 5 mm. in 

 the maximum of the band. If the reflecting power were only one-half 

 this amount (cf. the silicates), the galvanometer deflection after five 

 reflections would be one thirty-second of 5 mm., which could not be 



1 Rubens and Nichols, Ann. der Phys. (3), 60, p. 418, 1897. 



2 Aschkinass, Ann. der Phys. (4), I, p. 42, 1900. 



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