178 



ANNUAL KEPORT SMITHSONIAN INSTITUTION, 193 5 



SURFACE COMPOSITION 



We come now to the problem of ascertaining the nature of the 

 materials exposed at the surface of the moon. Obviously, we are 

 limited, in our approach to the problem, to a determination of the 

 effects which the materials have on sunlight on reflection. One 

 and a quarter seconds after the sun's rays leave the moon they reach 

 the earth. We can study and measure these reflected rays by dif- 

 ferent methods and compare them with direct rays from the sun. 

 We can also study and measure the changes produced in sun's rays 

 on reflection by terrestrial materials, such as rocks of various kinds 

 and other substances. These changes are not limited to the visible 

 spectrum, but include all the radiation received through the earth's 

 atmosphere from the ultraviolet into the infrared. The effects 



Figure 1. — Cliange in plane polarization of moonlight from maria. The curves show 

 the changes in percentage plane polarization of moonlight from different lunar maria 

 with change in phase angle. 



produced are of two kinds: A certain amount of plane polariza- 

 tion is introduced and different parts of the spectrum are reflected 

 to different degrees. Light is considered to be caused by vibrations 

 in a special medium. These vibrations take place, in free space, 

 at right angles to the direction of propagation. If the vibrations 

 are limited to a single plane, containing the direction of propaga- 

 tion and a line perpendicular thereto, the light is said to be plane 

 polarized. White light consists of vibrations of different fre- 

 quencies; it can be resolved into its component parts or frequencies 

 by the use of a spectroscope or spectrograph. The human eye is 

 sensitive to a small part only of the range of radiation frequencies; 

 this part is called the visible spectrum; those portions which are 

 beyond the power of the eye to sense are called the ultraviolet and 

 the infrared, respectively, when the frequencies are higher or lower 

 than the frequencies in the visible spectrum. 



