GEOPHYSICAIv LABORATORY. lOI 



lagh in Ireland. The work of these two investigators was remarkably thor- 

 ough and comprehensive and has served as the foundation on which all sub- 

 sequent researches have been based. Interest, however, has centered chiefly 

 in the reflection rather than in the refraction of light by crystal plates, and 

 much still remains to be accomplished on the details of the refraction prob- 

 lem. The present investigation was undertaken primarily to determine the 

 influence of certain factors which underlie the methods of optic axial-angle 

 measurement of crystal plates in the thin section, especially the method of 

 Professor Becke and Wright's modification of the same. These methods 

 are based on the degree of curvature of the dark hyperbolas or zero isogyres 

 of the interference figure, and depend, therefore, on the polarization direction 

 of light-waves transmitted along different paths. In microscopic work, the 

 influence of the boundary surfaces, not only of the crystal plate but also of 

 the intervening glass plates and lenses, on these waves enters the problem 

 and tends to render it exceedingly complicated. In this paper the general 

 mathematical treatment of the problem of light transmission through trans- 

 parent inactive crystal plates is given and several new and important rela- 

 tions are deduced which simplify the presentation materially. The results 

 of calculation are checked by series of observations on plates of calcite, mus- 

 covite, selenite, and quartz, with apparatus specially designed for the pur- 

 pose. The results of the investigation show that the methods proposed by 

 Professor Becke and by Wright are approximate methods only ; both furnish 

 results of about the same order of accuracy, the single advantage of Wright's 

 method being that of slightly greater simplicity. They show, furthermore, 

 that a theoretically correct method is not practicable, because of the many 

 complicated factors of only slight influence which would have to be taken 

 into consideration. Incidentally, the general effects of lenses on interference 

 figures are discussed; also the construction of nicol prisms and the adjust- 

 ment of the petrographic microscope. 



This rotating effect of the boundary surfaces on the planes of polarization 

 of transmitted light-waves underlies all observations involving the use of 

 convergent polarized light or of parallel polarized light entering the crystal 

 plate at angles with the normal. The influence is only slight in any case, 

 but it precludes a high degree of accuracy in any measurements based on 

 such phenomena. Methods involving the use of interference figures or of 

 the universal stage, although of great practical value, are only approximate 

 methods, and, because of the complexity of the factors involved, can not be 

 modified so as to be theoretically correct. The direct application of Fres- 

 nel's rule for finding the planes of polarization of transmitted waves is, in 

 such cases, not strictly correct ; but experience has shown that the error in- 

 troduced thereby is not greater than that resulting from other causes and is 

 therefore permissible. 



(19) Some glacial effects of ice action in Iceland. Fred. Eugene Wright. Bull. Geo!. 

 Soc. Amer. 1910. (In press.) 



A Study of the physiographic features developed by glacial erosion with 

 special reference to the difference between valley glacier erosion and that of 

 the continental ice sheet on the uplands, the one tending to develop downward 

 rather than laterally, and to accentuate differences in elevation, the other 

 serving as a plane of reference toward which exposed masses tended to be 

 reduced, the ultimate result of its action being to truncate all mountain 

 masses at a common level, the upland surface thus produced strongly re- 

 sembling in appearance an uplifted and dissected peneplain. 



