RECENT ADVANCES IN SCIENCE 463 



forming from the first set of axes to the second, or from the 

 second to the first, and thus, as stated above, it is impossible by 

 means of such relations to determine which of the two systems 

 of reference (if any) is at absolute rest ; they merely indicate 

 a relative motion of one system to the other with a velocity 

 uniform in amount and direction. The existence of these 

 relations leads to a number of results which seem paradoxical 

 at first to our preconceived ideas concerning space and time ; 

 thus it appears that when a body originally at rest in a definite 

 system of reference is set in motion, its dimensions measured 

 relative to axes in this system are contracted in the direction 

 of motion and unchanged in planes perpendicular to this ; its 

 dimensions relative to a system of reference moving with it are 

 the same as they were originally. But this result is precisely 

 the assumption which Lorentz and Fitzgerald introduced a 

 few decades ago to account for Michelson's failure to detect the 

 (assumed) influence of the Earth's motion on the velocity of 

 light. A further deduction from these relations concerns the 

 time registered by a clock between two definite occurrences. 

 If this interval be measured by two clocks each one fixed in 

 one of two systems of reference in relative motion, the two 

 observations will not agree ; in fact, if the pendulum of one of 

 the clocks could be observed from the other system, it would 

 appear to have its period increased beyond the value it has 

 when measured by observers at rest relative to it. The effects 

 referred to depend on the square of the ratio of the relative 

 velocity of the two systems to the velocity of light, and it 

 appears hopeless at present to obtain (say) two platforms 

 moving on the Earth's surface with a relative velocity large 

 enough to produce these anticipated results in large enough 

 magnitude to be measurable. Einstein's views and the four- 

 dimensional interpretation given to them somewhat later by 

 Minkowski present the concepts of space and time not as two 

 distinct modes of perception, but as two closely related aspects 

 of reality. Very lucid and sound accounts of the principle may 

 be found in Campbell's Modern Electric Theofy (2nd edition) 

 and Richardson's Electron Theory of Matter. A very complete 

 exposition is given in Cunningham's Principle of Relativity. 



The work with which the British Association meeting was 

 immediately concerned was the recent attempt of Einstein to 

 include gravitational phenomena within the scope of a " General- 



