CHAPTER I. 



INTRODUCTION. 



Previous work on this subject has been rather disconnected, and the 

 present investigation is only preliminary to an extended inquiry into the 

 subject. Nichols 1 compared the reflecting power of quartz with silver, 

 and found that from the visible to 8 p. only a small amount is reflected, 

 while at 8.5 and 9.02^ there are two intense reflection bands which 

 have a reflecting power almost as great as silver. This work was con- 

 tinued by Rubens and Nichols, 2 by using successive reflections from sev- 

 eral surfaces of the same mineral, who found the location of the maxima 

 of the selectively reflected rays("Reststrahlen")of a series of substances, 

 including quartz, mica, rock salt, sylvite, crown and flint glass, sulphur, 

 alum, and calcite. By using a wire grating they were able to extend 

 their observations to 61 p., the longest heat-waves yet discovered. 



In 1900, Aschkinass, 3 using a rock-salt prism, compared the reflecting 

 power of a series of minerals, including marble, calcite, selenite, and 

 alum. He found that marble and calcite have strong reflection bands 

 at 6.7 and 11.4^, while selenite has a band at 8.69^. The 6.67 band 

 of marble is broad, and no doubt complex ; and in the present work it 

 will be noticed that the calcite band is also double, with the stronger 

 component at 6.6 p.. 



Recently Porter, 4 using the method of Rubens and Nichols, found 

 the selectively reflected rays of a series of compounds for the region 

 extending to 10 p. By this method he was able to locate maxima which 

 are very small and would not appear by a single reflection. For 

 example, potassium ferrocyanide has a series of medium-sized bands 

 which were found in the present examination of absorption spectra, one 

 of which also appears in his reflection spectrum of this compound. 



APPARATUS AND METHODS. 



The apparatus for the absorption work was used in the present exam- 

 ination. To this end the Nernst heater was replaced by a sliding carrier 

 containing the reflecting mirrors (fig. 59). The carrier consisted of a 

 vertical sheet of metal which, by means of cords and pulleys, was moved 



1 Nichols : Phys. Rev., 4, p. 297, 1897. 



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

 'Aschkinass: Ann. der Phys. (4), I, 42, 1900. 



4 Porter : Astrophys. Jour., 22, p. 229, 1905. 



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