SURFACE FEATURES OF THE MOON — WRIGHT 



179 



Thus far we have used, and are still using, four different methods 

 for these measurements ; a visual method employing a special polariza- 

 tion eyepiece for the measurement of the amount of plane polarization 

 in the rays for different points on the moon's surface and at different 

 phases of the moon ; a photoelectric-cell method for the measurement 

 both of the amount of plane polarization and of the relative spectral 

 intensities of the rays ; a thermoelement method for the same purpose ; 

 and a polarization spectrograph. These methods require special appa- 

 ratus, devised or adapted to the problem in hand. The moon is an 

 unusually favorable object for the testing of new methods and appa- 

 ratus suitable for analyzing the characteristics of sunlight reflected 

 by a planet or satellite of the solar system. 



Figure 2. — Change in polarization iu moonlight from mountains. The curves show the 

 changes in percentage plane polarization of moonlight from different mountainous areas 

 witli change in phase angle. 



For the visual measurements a special eyepiece enables us to 

 ascertain the percentage plane polarization in a moonbeam accurate 

 to one-fifth of 1 percent. The field of the eyepiece is a divided 

 photometric field in which two factors, equality of illumination and 

 exact alignment of Savart fringes, are the two criteria used in mak- 

 ing a measurement ; it is the combination of these two factors which 

 renders the method so accurate. With the aid of this eyepiece 24 

 selected small areas on the moon have been studied and the amounts 

 of plane polarization in the reflected light measured for different 

 phases of the moon. The measurements extended over four luna- 

 tions, and nearly 10,000 individual readings with the new eyepiece 

 mounted on a 6-inch refracting telescope were made, so that we now 

 know with a fair degree of certainty the amount of plane polariza- 

 tion present in a beam of moonlight from any given area on the 

 moon at any given phase. The general results are shown in figures 

 1 and 2. On an average the mountains and lighter areas reflect 



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