APPARATUS AND METHODS. 75 



surfaces will be required. Hence the present examination is limited to 

 those minerals which afforded surfaces I by 2 cm. or larger. 



Before discussing the results it will be well to recall some of the 

 known facts in regard to absorption, reflection, and dispersion. In case 

 of normal dispersion the index of refraction decreases with increase in 

 wave-length. For a transparent substance this means a decrease in 

 reflecting power, with increase in wave-length. In the region of an 

 absorption band the refractive index will be abnormally decreased on 

 the side toward the short wave-lengths, while it will be abnormally 

 increased on the side of the long wave-lengths. Consequently the reflec- 

 tion curve, which is now a function of the refractive index and the 

 extinction coefficient, will be abnormally decreased on the side of short 

 wave-lengths and abnormally increased on the side of the long wave- 

 lengths. 



These facts are well illustrated in the present series of curves. The 

 same is to be observed in Aschkinass's curves. He makes no com- 

 ments, however, although the minerals were examined in connection 

 with the question of anomalous dispersion. In comparing the present 

 curves with the transmission curves it will be noticed that the maxima 

 do not coincide. This is to be expected, for in the transmission curves 

 in the region of great absorption and anomalous dispersion the loss of 

 energy is mostly by reflection, hence the maximum will be abnormally 

 broadened and shifted. This is well illustrated in the transmission 

 curve of potassium nitrate, which has a large opaque region extending 

 from 6.5 to 8.5 ju,. Before examining this region by reflection the 

 writer expected to find this region to be a complex of several bands, as 

 in calcite, or at least of one large band with a maximum at about 7.5 /*. 

 The reflection band, however, is found at 7.18/11, and the asymmetry in 

 the transmission curve is to be attributed to the slight loss of energy, by 

 reflection on the side of the short wave-lengths and to a correspond- 

 ingly great loss on the side of the long wave-lengths. 



Before discussing the reflection curves it may be added that the crys- 

 talline minerals were usually cleavage pieces cut (but not always pol- 

 ished) in the same direction as for the transmission work. A series of 

 sulphates will first be noticed. 



