270 Dr. S. Tolver Preston on Certain Questions 



In respect, to the physiography of the Moon : it may be 

 questioned whether the larger ring-like* formations are 

 really " craters " formed subsequently to the general solidifi- 

 cation of the lunar crust, in the sense in which the phrase 

 " crater ". is applied to a terrestrial volcano. On our globe> 

 it might not even occur to anyone to apply the name 

 " crater " to a circumferential ridge fifty to even a hundred 

 miles in diameter, — of breadth so disproportionate to height 

 that the opposite brink of the ridge might be expected to 

 be invisible, involving (by comparative estimate) some days 

 at ordinary walking pace to pass across the enclosed area. 

 Statistical figures may advantageously convert themselves 

 into adequate quantitative conceptions in the mind, if only 

 to achieve that just idea of existing facts, which becomes the 

 preliminary to any interest that reasonable speculations 

 (grounded on these facts) may present. 



Some appear to have a preconceived notion as to 

 there being something beyond possible intellectual pene- 

 tration in these inquiries. But there can be no motive for 

 ignoring the evidence of the spectroscope as to the intimate 

 physical relationship of Matter, in respect to many of its 

 constituents, within the range of our own universe at any rate ;; 

 while an intervening distance of 240,000 miles (that of our 

 satellite) would, in the career of an ocean liner, make but a 

 small record. The theory that the lunar globe was formed 

 out of the substance of our Earth, a case of physical " fission," 

 under a modification of the familiar Nebular Hypothesis, 

 is of scientific interest. 



December 20, 1906. 



Supplement. — A question of a somewhat different kind 

 might be mooted, as an Addendum. 



In the instance of the side-view reflecting " Newtonian '* 

 telescope, it may be found desirable to diminish the size of 

 the front mirror, with the obstruction of light by it (and the 

 attendant virtual constriction of aperture of the chief mirror 

 — augmenting defect of diffraction). This diminution of its 

 dimensions may be achieved by locating the front- mirror nearer 



* With but a comparatively insignificant sheltering atmosphere,, 

 possibly the impacts of some meteors would leave marks visible on the 

 lunar surface, and in the description in Newcomb's ' Popular Astronomy ' 

 we may notice the passage: — "It is very curious that the figures of 

 these inequalities in the lunar surface can be closely imitated by throwing 

 pebbles upon the surface of some plastic mass as mud or mortar " (p. 322). 



