242 CONTEMPORARY SCIENCE 



may be made so as to introduce into the equations the 

 gravitational field recognized by an observer on the earth 

 near the box; but this, obviously, would not be the gen- 

 eral gravitational field, because the field changes as one 

 moves over the surface of the earth. A solution found, 

 therefore, as just indicated, would not be the one sought 

 for the general field; and another must be found which 

 is less stringent than the former but reduces to it as a 

 special case. He found himself at liberty to make a 

 selection from among several possibilities, and for sev- 

 eral reasons chose the simplest solution. He then tested 

 this decision by seeing if his formulae would degenerate 

 to Newton's law for the limiting case of velocities small 

 when compared with that of light, because this condition 

 is satisfied in those cases to which Newton's law applies. 

 His formulae satisfied this test, and he therefore was able 

 to announce a "law of gravitation," of which Newton's 

 was a special form for a simple case. 



To the ordinary scholar the difficulties surmounted by 

 Einstein in his investigations appear stupendous. It is 

 not improbable that the statement which he is alleged to 

 have made to his editor, that only ten men in the world 

 could understand his treatment of the subject, is true. I 

 am fully prepared to believe it, and wish to add that I 

 certainly am not one of the ten. But I can also say that, 

 after a careful and serious study of his papers, I feel con- 

 fident that there is nothing in them which I cannot under- 

 stand, given the time to become familiar with the special 

 mathematical processes used. The more I work over Ein- 

 stein's papers, the more impressed I am, not simply by his 

 genius in viewing the problem, but also by his great tech- 

 nical skill. 



Following the path outlined, Einstein, as just said, 

 arrived at certain mathematical laws for a gravitational 

 field, laws which reduced to Newton's form in most cases 

 where observations are possible, but which led to different 

 conclusions in a few cases, knowledge concerning which 



