SURFACE FEATURES OF THE MOON WRIGHT 177 



purpose a second reflecting mirror, also of G7.5 feet focal length, is 

 placed at such a position that it views the moon from the direction 

 of mean libration and casts an image of it on a photographic plate 

 momited in a compartment beside the illuminated globe. The photo- 

 graphs thus produced are projections on the plane of mean libration ; 

 they fulfill the requirements of a map on a given scale. In order to 

 complete the series of maps showing the moon at different phases 

 we still need photographs taken with the 100-inch telescope and its 

 zero corrector lens. During the past 2 years the seeing conditions at 

 Mount Wilson have not been such that we could obtain photographs 

 of the quality desired for this purpose. The series of maps made 

 by this method will be independent of the personal factor and be 

 more valuable a century hence than at present. 



PHOTOGRAPH OF MOON ON GLOBE 



The projection of the moon positive on the magnesia-coated globe 

 gives a surprisingly beautiful and realistic representation of the 

 moon's surface. The correct and undistorted appearance of the 

 craters and other features near the edge of the moon's disk is of 

 great aid in the visualization of the surface relationships. In order 

 to make this globular representation more accessible, a glass globe 

 coated on the outside with photographic emulsion was substituted 

 for the magnesia-coated bronze globe and the moon negative pro- 

 jected on it, thus producing a moon transparency which is angle 

 true. The globe is frosted on the inside and illuminated by an elec- 

 tric bulb. The coating with photographic emulsion was done, 

 through the courtesy of Dr. C. E. K. Mees, by the Research Labora- 

 tory of the Eastman Kodak Co., and represents a new advance in 

 i3hotographic technique. A dozen of these globes have been pre- 

 pared; they Avill be useful to the moon committee in its physio- 

 graphic work later; they may also serve as exhibits of miniature 

 moons showing the moon at different phases. 



The committee has also devised a method for making a topo- 

 graphic map of the central part of the moon out to 45° from the 

 center and with contour intervals of 500 feet or 200 meters. For 

 the preparation of this map advantage is taken of the libration of 

 the moon to obtain stereoscopic images from which, in turn, the 

 relative elevations can be ascertained by applying the principle used 

 in areal mapping from airplane photographs; with this difference, 

 however, that in airplane mapping the surface of reference or datum 

 plane is a horizontal plane, whereas on the moon we are interested 

 in the elevations with reference to a mean spherical surface. The 

 apparatus has not yet been built, and we shall not stop to consider 

 details of the method. 



