ADVISORY COMMITTEE ON ASTROMOMY lOl 



(7) A systematic photographic surv^ey of the nebulae with a large 

 reflecting telescope is needed at the present time. Photographs 

 should be taken of all important nebula in the northern and south- 

 ern heavens, for comparison with similar photographs to be taken 

 in the future. 



(8) Investigations should be made with moderate and high dis- 

 persions on the spectra of southern stars in connection with the 

 problem of stellar evolution. A large reflecting telescope would be 

 needed for this work. 



In relation to the lines of investigation enumerated in the fore- 

 going, it may be said tliat all have interest in and for themselves ; 

 but in relation to (i ), (4), (5), (6), and (8), it may also be asserted 

 that they ha\'e a further and very great interest, because the full 

 value of work already accomplished at northern observatories in 

 those lines will not be realized until the corresponding southern 

 observations have been made. 



In addition to the lines of investigation we have specified, there 

 are many others which would be of a high degree of interest in re- 

 lation to a southern obser^'atory. In fact, there are very few activi- 

 ties in northern observatories— saving only experimental researches 

 for the sake of opening new lines of investigation — which do not 

 need to find their counterpart in southern observatories to an extent 

 very much greater than 1^ now possible for existing observatories in 

 that quarter of the world. 



Site. — Such an observatory should be south of the thirtieth par- 

 allel of south latitude, if possible, for the following reasons : 



(a) Because the elevation of the southern pole in all the researches 

 specified in the first part of this communication ought to be at least 

 thirty degrees above the horizon in order to secure the necessary 

 steadiness of atmosphere in measurement upon objects near that 

 poie. 



\b) Because the proper combination of meridian observations with 

 those of the northern hemisphere is best effected when the latitude 

 of the southern observatory is as nearly as possible that of the 

 northern. 



U) Meridian obser\-ations in right ascension as well as in declina- 

 tion require that the ' ' seeing ' ' shall be good for at least ten degrees 

 below the pole. 



(a) For an observatory which is to require prolonged and vigorous 

 work of measurement, extreme tropical conditions, such as might 

 seriously aftect the health of observers, should be avoided. 



