March 12, 1920] 



SCIENCE 



261 



flection of 1".9. Of course the detection of 

 such a minute deflection was an extraordinar- 

 ily difficult matter, so many corrections had to 

 be applied to the original observaitions ; but 

 the names of tlie men who record the conclu- 

 sions are such as to inspire confldence. Cer- 

 tainly any effect of refraction seems to be ex- 

 cluded. 



It is thus seen thait the formulae deduced by 

 Einstein have been confirmed in a variety of 

 ways and in a most brilliant manner. In con- 

 nection witlh these formulse one question must 

 arise in the minds of everyone : by what proc- 

 ess, where in the course of the mathematical 

 development, does the idea of mass reveal it- 

 self? It was not in the equations at the be- 

 ginning and yet here it is at the end. How 

 does it appear? As a maitter of fact it is first 

 seen as a Constant of integration in the dis- 

 cussion of the problem of the gravitational 

 field due to a single particle; and the identity 

 pi this ooastant with mass is proved when one 

 compares Einsitein's formulse with N^ewton's 

 law which is simply its degenerated form. 

 This mass, though, is the mass of which we 

 become aware through our experiences with 

 weight; and Einstein proceeded to prove that 

 this quanttity which entered as a constant of 

 integration in his ideally simple problem also 

 obeyed the laws of conservation of mass and 

 eonservation of momentum when he investi- 

 gated the problems of two and more particles. 

 Therefore Einstein deduced from his study of 

 gravitational fields the well-lcnown properties 

 of matter which form the basis of theoretical 

 mechanics. A further logical consequence of 

 Einstein's development is to show that energy 

 has mass, a concept with which every one now- 

 adays is familiar. 



The description of Einstein's method which 

 I have given so far is simply the story of one 

 success after another; and it is certainly fair 

 to ask if we have at last reached finality in our 

 investigation of nature, if we have attained to 

 truth. Are there no outstanding difficulties? 

 Is there no possibility of error ? Certainly, not 

 until all the predictions made from Einstein's 

 formulae have been investigated can much be 

 said; and further, it must be seen whether any 

 other lines of argimient will lead to the same 



coniolusions. But without waiting for all this 

 there is at least one difficulty which is ap- 

 parent at this time. We have discussed the 

 laws of nature as independent in their form of 

 referentee axes, a concept which appeals 

 jSti'ongly to our philosophy ; yet it is not at all 

 iclear, at first sight, that we can be justified in 

 our belief. We can not imagine any way by 

 which we can become conscious of the transla- 

 tion of the earth in space; but by means of 

 gyroscopes we can learn a great deal about its 

 rotation on its axis. We could locate the posi- 

 tions of its two poles, and by watching a Fou- 

 eault pendulum or a gyroscope we can obtain a 

 number which we interpret as the angular ve- 

 locity of rotation of axes fixed in the earth; 

 angular velocity with reference to what? 

 Where is the fundamental set of axes? This 

 is a real difficulty. It can be surmounted in 

 several ways. Einstein himse'lf has outlined a 

 method which in the end amounts to assuming 

 the existence on the confines of space of vast 

 quantities of matter, a proposition which is 

 not attractive. deSitter has suggested a pe- 

 culiar quality of the space to which we refer 

 our space-time coordinates. The consequences 

 of this are most interesting, but no decision 

 ,can as yet be made as to the justification of the 

 hypothesis. In any case we can say that the 

 difficulty raised is not one that destroys the 

 real value of Einstein's work. 



In conclusion I wish to emphasize the fact, 

 which should be obvious, that Einstein has not 

 attempted any explanation of gravitation; he 

 has been occupied with the deduction of its 

 laws. These laws, together with those of elec- 

 tromagnetic phenomena, comprise our store of 

 knowledge. There is not the slightest indica- 

 ,tion of a mechanism, meaning by that a pic- 

 ture in terms of our senses'. In fact what we 

 have learned has been to realize that our desire 

 to use such mecbanismis is futile. 

 I J. S. Ames 



The Johns Hopkins Univeksity 



LEARNED SOCIETIES, OLD AND NEW^ 



It would tax the younger men of science 

 beyond the compass of their imagination, if 



1 President 's address at the fourth meeting of" 

 the Annual Conference of Biological Chemists, held 



