GEOPHYSICAL LABORATORY. 131 



PUBLICATIONS. 



Brief reviews of the papers published by members of the laboratory 

 staff during the current year follow, together with synopses of some 

 that will appear at an early date: 



(1) Oblique illumination in petrographic microscope work. Fred. Eugene Wright. 



Am. Jour. Sci. (4), 35, 63-82. 1913. 



The study of interference phenomena resulting from oblique illumination 

 between crossed nicols enables the observer to determine many optical 

 features in a given mineral plate. These phenomena are identical, so far as 

 interference colors go, with the phenomena obtained in interference figures 

 from the same plate in convergent polarized light. The study of mineral 

 plates by the method of oblique illumination is of value because it impresses 

 the mind of the observer with the interdependence of optical and crystal- 

 lographic properties. It is, however, highly important that the observer 

 realize the essential agreement between the phenomena observed in oblique 

 illumination and those seen on interference figures in convergent polarized 

 light. In the interference figures the interference color phenomena are seen 

 at a glance, and if they be studied with reference to the position of the 

 mineral from which they are obtained, all of the conclusions to be ascer- 

 tained by means of the method of oblique illumination can also be derived 

 with even greater facility from the interference figure. For the study of 

 interference phenomena the method of oblique illumination does not offer 

 any special advantages over the convergent polarized light method, but it 

 does present certain disadvantages in manipulation and in the distinctness 

 of the phenomena observed which can not be disregarded entirely. This is 

 especially true if oblique illumination be obtained by use of a stop in the 

 eye-circle of the ocular, as recently suggested by Schneiderhohn. 



In this paper the phenomena produced by oblique illumination have been 

 discussed in some detail. Attention should also be directed to an obvious 

 but important fact, too often disregarded in petrographic microscope work, 

 that for the accurate measurement of extinction angles, central illumination 

 by parallel plane-polarized light is highly essential. Satisfactory measure- 

 ments of extinction angles can not be made when the section is illuminated 

 by a strongly convergent cone of light, incident under all angles and in all 

 azimuths. 



(2) An improved vertical-illuminator. Fred. Eugene Wright. J. Wash. Acad. Sci., 3, 



14-16. 1913. 



Of the two types of vertical-illuminators which are in current use on the 

 metallographic microscope, the reflecting-prism type furnishes the best 

 illumination with low-power objectives. With high-power objectives it is 

 less satisfactory, because the reflecting prism cuts off half of the rays from 

 the objective and thus seriously impairs the resolving power and general 

 efficiency of the optical system. The second type of illumination with the 

 Beck illuminator does not suffer from this defect, as the light which is 

 reflected from the thin glass plate passes, on its return after reflection from 

 the metal surface, through the glass plate itself on its way to the eye of the 

 observer. The glass plate thus serves both to reflect and to transmit the 

 light; the resulting intensity of illumination is, however, noticeably less 

 than that obtained by the first method. For satisfactory work the glass 

 plate should be plane and thin and the source of light so arranged that none 

 of the rays reflected from the objective lens surfaces reach the eye of the 

 observer; otherwise they cast a haze or fog over the entire field, thus reduc- 

 ing the contrasts and flooding the image with false light. 



