REPORT OF COMMITTEE ON OBSERVATORIES 35 



the correctness of the fundamental principle {b) or of some modifi- 

 cation of it which can only be had through direct measurement of 

 the distances of a large number of stars. 



That such measurements in large numbers are practicable is 

 demonstrated by the successful work of Gill and Elkin at the Cape 

 and of Elkin and Chase at New Haven with heliometers, by Kap- 

 teyn at lyciden and Flint at Madison (Wisconsin) with the transit, 

 as well as by other observers. 



Some of the available methods for measurement of parallax are : 



By the use of heliometers. 

 B}^ the use of meridian transits. 

 By photographic methods. 

 By micrometrical methods. 



All these methods have been tried extensively ; the first has ap- 

 parently proved most accurate ; the last, formerly employed almost 

 exclusively, would probably be discarded now by common consent. 

 Photographic methods have not hitherto proved entirely satisfactory 

 perhaps, and yet it seems to be the almost unanimous opinion of 

 those in the best position to judge that this method offers the greatest 

 promise for efficient work on a large scale when properly used. The 

 method of exposing the same plate at three successive phases of 

 parallax, suggested by Kapteyn some years ago, is the one which 

 seems to offer the greatest promise of economy in labor and precision 

 in the result. From existing evidence it does not appear that the 

 photographic method is likely to be very effective upon stars brighter 

 than the fifth magnitude. 



The heliometer would probably be better suited for parallax meas- 

 ures of bright stars. This method, though extremely precise, is 

 slow and costly. It has been employed at the Cape of Good Hope 

 in measurements upon a few of the far southern stars ; but, so far as 

 is known, no work of the kind is now going on in the southern hemi- 

 sphere. The Cape heliometer is now devoted to planetary observa- 

 tions upon a new plan, a work very appropriate to the original 

 purpose of the Royal Observatory at the Cape of Good Hope, 

 Therefore, with the exception of the aid which might hereafter be 

 rendered by the Cape heliometer, the entire field for the determina- 

 tion of stellar parallax in the southern hemisphere is open to the 

 Carnegie Institution, should it desire to enter it. 



This work could be undertaken with advantage on almost any 

 scale. Without any idea as to what means might possibly be avail- 



