ADVISORY COMMITTEE ON ASTRONOMY II 7 



and most effective observations of this class. Nothing of great im- 

 portance in this line of work has been accomplished in this country. 

 In this same line should be included the great photographic survey 

 now going on under the leadership of the observatory at Paris, 

 which has for its declared object the delineation upon photographic 

 charts of all stars down to the fourteenth magnitude in the entire 

 heavens. There are some indications of lagging in this, the main 

 feature of the cooperative plan described, owing to the great diffi- 

 culties in execution and the very great estimated cost of reproduction 

 of the plates upon printed charts. This country has taken no share 

 in this great work ; and this may be considered a matter of regret. 

 Undertakings of this kind indicate far-seeing insight into the needs 

 of the astronomy of the future, and a spirit of enterprise in the 

 breaking of new paths of investigation. Even the mere counting of 

 stars by Herschel (his " star-gauges " ) was the foundation of a new 

 philosophy of the heavens — one of the most fruitful chapters in our 

 science. How much greater things may be expected from the 

 charting of the whole heavens upon a uniform plan may easily be 

 imagined. This plan is most interesting from what we know it will 

 accomplish, and not less interesting from the unknown possibilities 

 which may flow from such works. 



Fundamental Star Positions. 



At the other extreme of the scale is found the determination of 

 the positions of the principal stars — fundamental or absolute obser- 

 vation, as it is called. This problem is twofold. First, we have to 

 find an invariable line of reference, or direction, in space — philo- 

 sophically one of the most interesting problems which has been 

 attempted in the whole range of exact sciences. The earth is rotat- 

 ing on its axis, with the direction of its axis continually disturbed 

 by a multitude of external attractions and terrestrial perturbations. 

 At the same time, the earth revolves in its orbit about the Sun sub- 

 ject to a multitude of perturbations disturbing uniformity of motion. 

 The earth has a third motion with the Sun, at a high velocity, 

 through space in a direction now approximately known. The prob- 

 lem is to determine with humanly absolute exactness the direction 

 of some line in space in such a manner that it can be identified with 

 certainty at any future time. 



The second problem of fundamental astronomy is to ascertain wdth 

 the highest precision what may be termed the graduation of the sky. 



