INTERCONNEXION OF ITS BRANCHES. 227 



It is not simply that, as we have already shown, those geo 

 metrical and mechanical principles by which celestial ap 

 pearances are explained, were first generalized from terres 

 trial experiences ; but it is that the very obtainment of cor 

 rect data, on which to base astronomical generalizations, 

 implies advanced terrestrial physics. 



Until after optics had made considerable advance, the 

 Copernican system remained but a speculation. A single 

 modern observation on a star has to undergo a careful anal 

 ysis by the combined aid of various sciences has to be digest 

 ed by the organism of the sciences which have severally 

 to assimilate their respective parts of the observation, be 

 fore the essential fact it contains is available for the further 

 development of astronomy. It has to be corrected not 

 only for nutation of the earth s axis and for precession of 

 the equinoxes, but for aberration and for refraction ; and 

 the formation of the tables by which refraction is calculat 

 ed, presupposes knoAvledge of the law of decreasing density 

 in the upper atmospheric strata ; of the law of decreasing 

 temperature, and the influence of this on the density ; and of 

 hygrometric laws as also affecting density. So that, to get 

 materials for further advance, astronomy requires not only 

 the indirect aid of the sciences which have presided over 

 the making of its improved instruments, but the direct aid 

 of an advanced optics, of barology, of thermology, of hy- 

 grometry ; and if we remember that these delicate obser 

 vations are in some cases registered electrically, and that 

 they are further corrected for the &quot; personal equation &quot; the 

 time elapsing between seeing and registering, which varies 

 with different observers we may even add electricity and 

 psychology. If, then, so apparently simple a thing as as 

 certaining the position of a star is complicated with so 

 many phenomena, it is clear that this notion of the inde 

 pendence of the sciences, or certain of them, will not hold. 



Whether objectively independent or not, they cannot 



