40 CARNEGIE INSTITUTION 



numerous other facts seem to lend a special significance to this 

 branch of astronomy. Except incidentally, this work is not very 

 closely associated with the stellar problem, and the immediate ad- 

 vantage of a greater extension of work in this line in the southern 

 hemisphere is not so apparent ; yet it should be remembered that 

 no really large telescope has yet been applied for any great length 

 of time to the measurement of double stars in the southern hemi- 

 sphere, and while we put this project after the four already men- 

 tioned, we entertain no doubt of its desirable character. 



It would seem very desirable that a telescope of about 27 inches 

 aperture should be provided for this work. There should be a reg- 

 ular survey of the entire southern sky for the discovery of new 

 double stars, to complete similar surveys carried out at the Lick 

 Observatory and elsewhere. 



Double-star work on the southern sky has come practically to a 

 standstill, while it is still going on industriously at several observa- 

 tories in the northern hemisphere. That a large telescope in the 

 southern hemisphere should be devoted to double-star measurements 

 for a period of about eight years seems to be evident. 



One observer and one assistant would be required. 



If a large refracting telescope should be provided for the proposed 

 Southern Observatory it is very likely that certain micrometrical 

 and other studies, apart from double-star observations, would be 

 worth while, perhaps calling for the detail of another observer. This 

 comparatively small expenditure would doubtless be well compen- 

 sated in the increased utility of a costly telescope justified for an- 

 other purpose. 



Possible combination of '(^) and (5). — In providing the instrumental 

 means for carrying out (3) and (5) upon a restricted scale, it might 

 be possible to combine the two telescopes required, so that one 

 mounting and observing room would suffice. This would result in 

 a very large saving in plant. The tubes of the two telescopes, each 

 designed as perfectly for its peculiar purpose as if made for special in- 

 struments, could be attached to the same declination axis. Parallax 

 plates should be exposed in the early evening and late morning 

 hours, in any case. The result would be that the parallax telescope 

 would usually be unused during the four or five hours around mid- 

 night, in order to avoid the use of the telescope at large hour angles 

 east or west. During this period, if the parallax telescope were 

 combined with the telescope for micrometrical observations the latter 

 would be wholly free during four or five hours near midnight, the 



