282 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 12 



in common use by petrologists, such as the sensitive-tint quartz plate, the Biot- 

 Soleil sensitive-tint biplate, the Bertrand eyepiece, and the biquartz- wedge 

 plate. Of these, the last is the most sensitive, because in it the sensibility is 

 variable and can be adjusted to meet the conditions of illumination. 



In opaque substances the precision attainable by these methods is, in general, 

 small, and the phenomena which can be observed are relatively few and re- 

 stricted in scope. As a result, one can not expect from the application of 

 polarized light to such substances the harvest of optical data which has been 

 gathered from transparent crystals. F. E. W. 



PHYSICS. — The measurement of the intensity of transmitted and reflected 

 light by polarization photometers. Fred E. Wright. Joum. Opt. Soc. 

 Amer. 2: 65-75. 1919. (Geophysical Lab. Papers on Optical Glass, 

 No. 22a.) 

 In this article a brief statement is given of the methods used by the writer 

 during the war period for the measurement of the light transmission of optical 

 glasses and of optical instruments. Several new attachments and im- 

 provements on the Koenig-Martens photometer are described ; also the method 

 for their use in the practical measurement of the amount of light transmitted 

 and reflected by optical glasses, and of the light transmission of optical 

 instruments. F. E. W. 



PHYSICS. — Polarization photometer prisms. Fred E. Wright. Journ. 

 Opt. Soc. Amer. 2: 93-96. 1919. (Geophysical Lab. Papers on Opti- 

 cal Glass, No. 226.) 

 In this paper is considered the quantitative effect of external and internal 

 reflections on the intensity of light-waves transmitted by the calcite rhomb 

 and the Wollaston prism, when these are used in photometric work. This 

 discussion is necessary to an adequate understanding of polarization photom- 

 eters and the factors underlying their use. F. E. W. 

 GEOLOGY. — The future of Alaska mining. Alfred H. Brooks. U. S. 

 Geol. Survey Bull. 7 14-A. Pp. 56, pis. 3, fig. 1 . 

 Although many local factors affect the future of Alaska mining, the most 

 important consists of the mineral reserves. An estimate of Alaska's mineral 

 reserves would be difficult enough, even with complete geologic maps. Only 

 twenty per cent of the Territory has been covered by even reconnaissance 

 geologic surveys, and less than one per cent by detailed surveys. The 

 information at hand does not permit of even approximate quantitative 

 estimates of reserves. It indicates, however, the areal distribution of the 

 mineral deposits and a study of their geologic occurrence gives a basis for 

 forecasting their availability to the miner. 



The wide distribution of placer gold in Alaska and the known occurrence of 

 many gold-bearing quartz veins shows the large areal extent of the auriferous 

 mineralization. Furthermore, the wide distribution of intrusive granites, 

 with which the occurrence of gold is genetically related, augurs well for future 

 discoveries. The Alaska placers have during 40 years of mining produced 

 $218,000,000 worth of gold. A careful consideration of all the facts avail- 

 able indicates that the placer gold reserves have a value of at least $360,000- 

 000. 



The Alaska copper deposits are widespread and are found in a number of 

 different modes of occurrence and geologic relations. It is important to 

 note that the Alaska copper deposits thus far developed are primary. The 

 practical deduction from these facts is that no greater variation in the mineral 



