180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



more light and contain approximately half as much plane polarized 

 light as the light from the maria and other dark areas. The maxi- 

 mum polarization occurs at the lunar phase angles, 100° to 110° 

 and 280° to 290°, and attains the maximum value of 16 percent in the 

 case of 1 or 2 maria. The plane of vibration is commonly nor- 

 mal to the plane of incidence; but near full moon the polarization 

 is negative and the plane of vibration is in the plane of incidence. 

 At phase angles, ±22° to 23°, the polarization is zero for practi- 

 cally all points on the moon's surface. It is also zero for phase 

 angles 0° (full moon) and 180° (new moon). This negative polar- 

 ization, first discovered by Lyot, attains a value roughly of 1 per- 

 cent as a rule. It is an abnormal phenomenon and is probably 

 due to diffraction and scattering. It is also observed on terrestrial 

 materials. 



Measurements of the percentage amounts of plane polarization 

 in sunlight reflected by terrestrial materials are being made with the 

 new eyepiece; they are not yet complete. When finished, they will 

 enable us to group the materials according to this property and thus 

 to ascertain with a fair degree of probability the nature of the lunar 

 surface materials. We know from measurements made with the less 

 accurate predecessor to this eyepiece that dark, opaque rocks and 

 other substances polarize the light more or less completely at certain 

 phase angles; whereas light-colored rocks and materials, into which 

 the light can penetrate and be reflected, polarize the light relatively 

 little, thus indicating that the lunar surface materials are of the 

 latter type. Additional evidence that the surface materials are of 

 the nature of volcanic ashes and pumice, high in silica, is given by 

 the rate of cooling of the moon's surface during an eclipse. As the 

 earth's shadow passes over the moon its surface temperature drops, 

 in the course of an hour, from +120° C. to below —100° C, accord- 

 ing to measurements by Pettit and Nicholson of Mount Wilson Ob- 

 servatory. This signifies, as computations by Dr. Epstein of our 

 committee show, that the lunar surface materials are exceedingly 

 good heat insulators; in other words, they have very small heat 

 capacity, are poor heat conductors and cannot, therefore, be massive 

 materials, like granite or limestone, but rather light siibstances re- 

 sembling, in characteristics, pumice and volcanic ashes. 



Measurements by the three other methods, photoelectric cell, 

 thermoelement, and the polarization spectrograph, are now in prog- 

 ress. In these three methods the special apparatus is mounted on a 

 20-inch reflecting telescope and the light from a given small area on 

 the moon is received on the light-sensitive receiver. The photo- 

 electric cell attachment consists of a special large compound Wolla- 

 ston prism of quartz in a rotatable mount, a vacuum potassium Kunz 



