CHAPTER I. 



INTRODUCTION. 



In the preceding volume, Part IV, the reflection spectra of several 

 groups of chemically related minerals were examined, including sulphates, 

 sulphides, and an extensive group of silicates. This examination did not 

 include the oxides and carbonates which were not obtainable at that time. 

 To examine the substances in chemically related groups seemed to be the 

 only logical way to gain insight into the mechanism of selective absorption 

 and of selective reflection. The main difficulty lay in obtaining material 

 of sufficient size to produce a suitable reflecting surface. A list of minerals, 

 which occur in sufficient size and homogeneity, was sent out to various 

 mineral dealers who very kindly submitted several large boxes of specimens 

 for selection. Even after this first elimination it was possible to obtain 

 only about 10 per cent of the number of minerals desired. In addition to 

 these minerals a considerable number of specimens were selected from the 

 collection in the U. S. National Museum. 



The apparatus and methods employed were essentially the same as in 

 the preceding investigation. The spectrum was produced by means of a 

 rock-salt prism, and mirror spectrometer previously described. The re- 

 flecting power of the mineral was compared with that of a silver mirror. 

 The surfaces were ground plane, but were not always of the highest polish. 

 Since primarily we are only concerned with the accurate location of bands 

 of selective reflection, the question of polish is of secondary importance. 



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

 preceding work. The radiation from a Nernst "heater" was projected 

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

 15 cm. and an aperture of 12 cm. 



The reflection spectrum was explored by means of a Rubens thermo- 

 pile. Although its sensitiveness was greater than the radiometer used 

 previously, the unsteadiness at times, due to magnetic disturbances, neces- 

 sitated repeating the observations at each spectrometer setting. When 

 using the radiometer, it was rarely necessary to repeat the observations in 

 exploring the spectrum up to 9 or 10 fi, while in the present work, using 

 a thermopile, the region beyond 9 p. was examined under the greatest 

 difficulties, and in the case of the carbonates no attempt was made to 

 locate the band at 11.4^. When the reflection faces were less than the 

 standard size, 2 by 3 cm., which was frequently the case, the silver com- 

 parison mirror and the specimen were covered with diaphragms having 

 equal openings, so that equal areas of the two surfaces were exposed. 



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