36 THE ADVANCE OF SCIENCE 



enlarged. Or, to put the matter in an- 

 other way, a doctrine which is untrue ab- 

 solutely, may, to a very great extent, be 

 susceptible of an interpretation in accord- 

 ance with the truth. At a certain period 

 in the history of astronomical science, the 

 assumption that the planets move in cir- 

 cles was true enough to serve the purpose 

 of correlating such observations as were 

 then possible ; after Kepler, the assump- 

 tion that they move in ellipses became 

 true enough in regard to the state of ob- 

 servational astronomy at that time. We 

 say still that the orbits of the planets 

 are ellipses, because, for all ordinary pur- 

 poses, that is a sufficiently near approxi- 

 mation to the truth ; but, as a matter of 

 fact, the centre of gravity of a planet de- 

 scribes neither an ellipse or any other 

 simple curve, but an immensely compli- 

 cated undulating line. It may fairly be 

 doubted whether any generalisation, or 

 hypothesis, based upon physical data is 



