GEOPHYSICAL LABORATORY. 167 



especially that part of the theory which treats of the reflection phenomena 

 resulting from vertically incident light-waves under the conditions usually 

 encountered in the use of the reflecting or metallographic microscope. Nor- 

 mally incident white light contains, after reflection by an anisotropic sub- 

 stance, a certain amount of plane-polarized light, and this amount increases 

 with the strength of the birefringence and the biabsorption in the crystal 

 plate. The presence of plane-polarized hght in natural light can be detected 

 by several different methods, such as are used in determinations of sky 

 polarization. For this purpose Koenigsberger adopted the Savart method 

 with rotating glass compensator. A second and new method is proposed 

 which employs either a single calcite cleavage plate with proper aperture or a 

 small portable Koenig-Martens photometer. This method is more sensitive 

 than the first. Methods of this kind, which are based on differences in in- 

 tensity of the reflected components of vertically incident light, are fifty or 

 more times less sensitive in the detection of anisotropism than methods based 

 on the phenomena produced by plane-polarized transmitted light-waves. 



In case vertically incident, plane-polarized Ught is used, the difference in 

 amphtude of the reflected components causes a rotation of a plane of polariza- 

 tion, and this can be detected and measured by any one of a number of devices 

 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 relative few and 

 restricted in scope. As a result, one can not expect from the appUcation of 

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

 gathered from transparent crystals. 



(3) The measurement of the intensity of transmitted and reflected Ught by polarization 



photometers. Fred. E. Wright. J. Opt. Soc. Amer., 2, 65-75 (1919). (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 Ught transmission of optical 

 instruments. 



(4) Polarization photometer prisms. Fred. E. Wright. J. Opt. Soc. Amer., 2, 93-96 (1919). 



(Papers on Optical Glass, No. 22b.) 



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 necessarj' to an adequate understanding of polarization photo- 

 meters and the factors underl^dng their use. 



(5) The contrast sensibility of the eye as a factor in the resolving power of the microscope. 



Fred. E. Weight. J. Opt. Soc. Amer., 2, 101-107 (1919). 



In this paper three factors of importance in high-power microscope work 

 are emphasized, namely: (a) The use of a polarizing prism to eliminate that 

 part of the field-hght which does not contribute to the diffraction pattern in 

 the image and hence tends to reduce the contrast and to decrease the sharp- 

 ness and crispness of the image. This phenomenon arises because diffracted 

 beams, which emerge from gratings whose interval is of the order of magnitude 



