REPORT OF COMMITTEE ON OBSERVATORIES 1 23 



A, 2. There are two other problems of the older astronomy which 

 cry out for solution, one of which is a comparatively small affair, 

 the other a very big one, but both are urgent. I refer to the com- 

 pletion of the organization in the southern hemisphere for deter- 

 mining change of latitude (that is the smaller affair) , and to the 

 formation of a parallax Durchmusterung — i. c, determining the 

 parallaxes of all the stars to a certain order of magnitude — (that is 

 the large one). 



I take the simpler matter first. Mr. Chandler proposes a southern 

 belt of observatories : 



Lat. Long. 



Sydney — 33 51' — 151 2' 



Cape of Good Hope — 33 56 — 18 5 



30 miles south of Santiago — 33 54 + 70 7 



It should involve very little trouble and comparatively small ex- 

 pense to establish the necessary organization at Sydney and the 

 Cape, and I venture to think that the Carnegie Institution could not 

 be better advised than to provide at once for the observatory near 

 Santiago, equipped with two observers devoted wholly and solely to 

 determination of the aberration constant and change of latitude. 

 The instrument I would recommend for all the three observatories 

 would be the photographic almucantar and the method used by 

 Cookson (see Monthly Notices, R. A. S., LXi, p. 315). There 

 should be observations of every group of stars in the early evening, 

 the early morning, and near midnight, at all times when opportuni- 

 ties occur. In this way we ought to get an extremely accurate de- 

 termination of all the latitude changes and a powerful determination 

 also of the aberration constant. 



A, 3. The parallax Durchmusterung is a very much greater un- 

 dertaking, but it is of the supremest interest to science. I do not 

 think it desirable to go beyond magnitude g}4 or perhaps 9 ; even 

 then the taking and measuring of the plates is a very big business 

 and involves a large organization. 



A telescope of large aperture is not necessary, but considerable 

 focal length is requisite to give the necessary precision of meas- 

 urement. The highest optical perfection should be arrived at, 

 probably a 4-glass objective of 8 or 10 inches aperture and 20 feet 

 focus would be most suitable. I think it very undesirable to employ 

 a ccelostat or any plan involving reflection from a plane mirror, as 

 plane mirrors may be liable to flexure or deformation by tempera- 

 ture changes. 



