Prof. N. S. Shaler on Earthlight on the Moon. 12a 



fingers/' As the comparative amounts of mental exertion re- 

 quired for these operations remain undetermined^ it is an open 

 question which requires the greater; but the fact that Dr. Wright 

 compares the mental help which is afforded by a theory to that 

 of the mechanical process of counting on the fingers, explains in 

 a more forcible manner than I can why he charges the atomic 

 theory with being unnecessary. 



It appears that Dr. Wright would have the man of science 

 erect his edifice with facts only; but I imagine, were he a builder, 

 he would wish for something more than bricks. Also that 

 he would deprive the .investigator of using the power of imagi- 

 nation — one of the greatest possessions of an intellectual being ; 

 but were he to examine the work which has been done in che- 

 mistry and physics, above the grade of the artisan, he would 

 find such evidence of the value of theory or the power of imagi- 

 nation that he, whom I know to be actuated by love for our 

 science, would be sorry to see chemists debarred from the legi- 

 timate use of their mental instrument of research. 



Were the anti-atomists to attack the object of their aversion by 

 experiment, no one could doubt their sincerity, and in all proba- 

 bility much good would result to science ; but to attack any 

 theory in the worn-out style of certain writers prior to the time 

 of Bacon, not only is productive of waste of energy, but may give 

 rise to the opinion among science-readers that the men who know 

 how to employ and keep a theory in its proper place necessarily 

 believe in its being true. 



73 Artesian Road, Bayswater, W. 



XVI. Earthlight on the Moon. By N. S. Shaler, Professor 

 of Palceontology, Harvard University. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



YOUR correspondent, Mr. Samuel Sharpe, in a note *' On 

 the Moon seen by the naked Eye,^' in the Number for 

 June, seems to have fallen into an old and often-corrected error 

 concerning the origin of the faint light which may be seen over 

 the dark part of the moon during the time when the bright part 

 is limited to a narrow crescent*. That ashy light, 'Humiere 

 cendree" as it has been called by the French, is clearly caused 

 by the earthshine or light of the sun -lit earth reflected back to 

 us. It would hardly be worth while to call attention once again 

 to one of the simplest of the phenomena of our satellite, were it 



- * The true cause of this appearance was first recognized by Leonardo da 

 Viuci, and afterwards by Moesthn. 



