NATURAL PHILOSOPHY. 157 



quite dense enough to support terrestrial life and vegetation at the 

 average level of the lunar surface. It gives no proof that such an at- 

 mosphere exists, but does give very good reasons why, if there be one, 

 we have failed to detect it with any certainty. 



ADDITIONAL RESEARCHES ON THE FIGURE OF THE MOON. 



No one who has seen Mr. De la Rue's (London) stereoscopes of the full 

 moon, in which the two images are obtained separately, but by one and 

 the same optical instrumentality at the epochs of her extreme eastern 

 and western librations in longitude, according to Mr. Wheatstone's in- 

 genious suggestion, can fail to have been struck by the marked and un- 

 deniable deviation from the spherical form which the double picture 

 suggests, standing out, as the convex surface does, in bold and full re- 

 lief; exhibiting the most complete appearance of a round, projecting 

 (vaguely speaking), globular figure. It is quite obvious, in a certain 

 mode of presenting the images to the eyes, that, were it really a solid 

 object so presented to our view, no one would hesitate to pronounce it 

 rather egg-shaped than spherical. The apparent curvature of the sur- 

 face under such circumstances is not that of a perfect sphere, alike 

 throughout ; but conveys the irresistible impression of an elongation in 

 one direction, and that, not directly toward the eye, but forming a 

 pretty considerable angle, with the visual ray joining the eye and the 

 moon's centre. Nor does the form even present a perfect symmetry, as 

 of a solid of revolution ; but, on the contrary, somewhat distorted, or, as 

 it were, skewed. The question which now arises is, how far any such 

 appearances in a stereograph are to be received as evidence of a cor- 

 responding reality of conformation in the moon itself. And here we 

 must at once reject any idea of explaining them by optical distortion, 

 due to instrumental causes, or to photographic error, or subsequent dis- 

 tortion in procuring the positive impressions from the original negatives. 

 The instrumental means at Mr. De la Rue's command preclude the one 

 supposition, and the photographic process employed (collodion on glass, 

 optically copied), the other. 



Mr. Gussew, Director of the Imperial Observatory at Wilna, with a 

 view to determine how far the whole or any part of this apparent 

 anomaly of figure is real, has subjected each of the two pictures of a pair 

 in his possession, given him by Mr. De la Rue, to careful and rigorous 

 microscopic measurement, by selecting on each of the pictures a con- 

 siderable number of sharply-defined, and securely identifiable points, 

 identical in each, and by measuring with extreme precision, by the aid 

 of an apparatus constructed for the purpose, their distances from the 

 centres and several points in the circumferences of the pictures. From 

 these measures (which under such circumstances must be regarded as 

 fully entitled to all the confidence of microinetrical measures, astro- 

 nomically taken at the telescope), on subjecting them to mathematical 

 computation, and applying the necessary corrections for parallax and 

 refraction as affecting the diameter of the moon, and the apparent figure 

 of her disk. Mr. Gussew has been led to conclude that a real eccen- 

 tricity of the figure actually does exist, and that, in point of fact, a por- 

 tion of the moon's surface having its axis directed about five degrees 

 from the earth as seen from the moon at the epoch of her mean libration, 

 may be considered as belonging to a sphere of smaller radius (and, 

 14 



