FUNDAMENTAL CONCEPTIONS 369 



tested through investigation founded on the observed motions of the 

 stars. The problem of finding a fixed line of reference in direction 

 that can be identically recovered at any future time, so far as this 

 can be accomplished under its necessary limitations, has received a 

 marked degree of attention, and will receive much more. 



In fact, the beginnings of actual research upon the problem itself 

 have been made in the discovery of the sun's motion of translation in 

 space. This discovery, with the operations that have established it, is 

 roughly analogous to the work of Hipparchus in determining a nom- 

 inal orbit of the sun. The solar motion appears likely to furnish us 

 with the base-line needed in stellar investigation; and this naturally 

 becomes an object of close attention until the outline of facts-regard- 

 ing it shall be well determined. The sun is laying down this base-line 

 at an annual rate of distance which is probably from four to six times 

 as great as that which separates the earth from the sun. It is evident 

 that the length of this base-line rapidly accumulates with time. Al- 

 ready a sufficient length of it has been paid out, so that we see the 

 nearer stars from a distinctly different point of view. We are even 

 now able to determine the relative mean distances from us of stars 

 classified in groups. It is even now almost possible to execute the 

 triangulation from this base by means of which in connection with 

 spectroscopic measures of radial motion, we may determine the dis- 

 tances of individual stars. The facility with which this can be done will 

 increase in a ratio more rapid than that of the increased lapse of time. 

 Within a century from now we may anticipate that astronomers will 

 begin to see the stars in space of three dimensions, and that this ability 

 to see them thus will, thereafter, rapidly become more clear and un- 

 doubted. 



There is now at the disposal of research in stellar astronomy an 

 accumulation of carefully measured positions of stars at desired epochs 

 in the past much larger than might have been provided on account 

 of anticipated needs in this special line. The requirements of geodesy 

 and of the astronomy of planets and comets are largely responsible 

 for this. Instead of a supply of observations growing out of the pre- 

 viously declared needs of the stellar problem, we find interest in that 

 problem fostered and stimulated by the opportunity for investigation 

 afforded by observations mainly provided for another purpose. 

 Astronomy is filled with illustrations of the fact that its progress is 

 frequently due to a natural development, rather than to deliberate 

 plans in advance. Thus Bradley, in the eighteenth century, set out 

 to detect the minute apparent motions of stars due to the already 

 predicted effect of nutation. While engaged in this investigation he 

 discovered the effect of aberration; and these two discoveries further 

 led to a complete revolution in the standards of accuracy which it 

 became feasible to prescribe for astronomical measurements. Again, 



