REPORT OF COMMITTEE ON OBSERVATORIES 1 37 



same material for the reduction of the remaining parts of the south- 

 ern sky. Therefore, though (with a view to the indefinite time 

 which may still elapse before we get a complete catalogue of right 

 ascension and declination for use from the ' ' Carte du Ciel " ) I 

 would most cordially rejoice in an extension of the astrographic 

 catalogue, I would still place this work in the second rank of the 

 works most urgently demanded at the present moment. 



However this may be, I will restrict my remarks to the three works 

 mentioned, which are the most important of all. 



(i) Determination of Parallax Soiith of Decli?tation — 20° . 



As, in my view, there is at present no work so urgently demanded 

 for the advancement of astronomy as the determination of parallax 

 on an extensive scale, the equipment of the obser\'-atory for this 

 purpose should be as complete as possible ; for instance : 



a. Two photographic telescopes, say of 40 cm. aperture and 6 

 meters focal length. They ought to give round images over a field 

 of 2° diameter. 



b. One telescope of 40 cm. aperture and a focal distance as great 

 as is compatible with rigid mounting. A round field of, say, 80' 

 diameter or even somewhat less will be sufiicient. 



c. One transit instrument of 7 inches opening. 



d. One heliometer of 7 inches. 



Regular morning and evening obser^'-ations demand two observ^ers 

 for each instrument. A small part only of the time of these ob- 

 servers would be taken up by the observations ; the rest would be 

 devoted to the measuring of the plates and the reduction of the 

 observations. 



To provide such an outfit and such a staff exclusively for the pur- 

 pose of parallax determination may seem extravagant. I do not 

 think so. The need of a better and more solid knowledge of stellar 

 distance is so great that we should stick to some such plan as 

 is involved in the above, even if it appeared that thereby the 

 funds available would be exhausted. If something must be sacri- 

 ficed, I think the instruments c and d could be best dispensed with, 

 as it seems more likely that the observations to be made by these 

 instruments will be taken up elsewhere. 



The two telescopes a would serve for a photographic Durchmus- 

 terung for parallax. Elsewhere (Publications Astr. Lab. Gronin- 

 gen No. I, pp. 87-98) I have explained at some length the feasi- 

 bility and desirability of such a plan. 

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