SURFACE FEATURES OF THE MOON — WRIGHT 181 



photocell of fused quartz, and the special amplifying circuit of 

 DuBridge and Brown adapted and improved by Dr. Stebbins and 

 employing the new electrometer tube D-96475, of the Western Elec- 

 tric Company. A more refined apparatus of this type is employed 

 by Dr. Stebbins in his work with the photoelectric cell on the stars 

 and nebulae. The thermoelement is of the vacuum type, made by 

 Dr. E. Pettit, and is equipped with the rotatable compound 

 Wollaston prism of quartz ; like the photocell, it is used together with 

 ray filters to isolate certain parts of the spectrum. The thermoele- 

 ment is not nearly so sensitive as the photocell, but it extends over 

 the entire spectrum and is useful as a check on the other measure- 

 ments. The polarization spectrograph is of the ultraviolet type, but 

 also serves throughout the visible spectrum. In the parallel beam 

 between the collimator and the first prism a Wollaston prism of 

 quartz in a sliding mount can be inserted and two spectra obtained, 

 the one with vibrations in the plane of incidence and the second with 

 vibrations normal thereto. Approximately 200 spectrograms of dif- 

 ferent parts of the moon were taken with this spectrograph during 

 the past summer. The spectrograms yield information both on the 

 percentage polarization for any given wave length and on the rela- 

 tive intensities for different w^ave lengths. Although not so sensitive 

 as the photoelectric cell, the polarization spectrograph covers a much 

 wider range of wave lengths, through the ultraviolet into the deep 

 red of the visible spectrum. 



PROGRAM FOR FURTHER WORK 



We plan to complete these measurements within the next 2 or 3 

 years; also to measure the changes in polarization of total moonlight 

 with change in phase and for different parts of the spectrum; also 

 the change in total intensity of moonlight with change in phase; 

 also to obtain additional photographs of satisfactory quality to en- 

 able us to proceed with the preparation of the photographic lunar 

 map. We are working along quite definite lines with apparatus and 

 methods developed in detail. We shall gather data of measurement 

 which should enable us to ascertain with fair certainty what the 

 lunar surface materials are and how they are disposed over the sur- 

 face of the moon insofar as it is visible to us. With that informa- 

 tion available, together with a good lunar map and a knowledge of 

 the conditions existing at the surface of the moon, we shall be in a 

 position to attack the problem of the physiography and mode of 

 formation of the lunar surface features. 



The questions arise : Why should a problem of this sort be solved ? 

 Why should a scientist give his time and energy to their solution? 

 These are proper questions and they should be faced. The scientific 



