liv Proceedings of the South African Philosophical Society. 



flexure (vertical or lateral) of the tube of the telescope or of the axis 

 on which the tube is mounted. ■ There must be means for eliminat- 

 ing these errors, and this can be best done by providing means for 

 reversing the telescope, pivot for pivot, and eye end for objective end, 

 on the same tube, so that in the mean not only these errors, but the 

 errors of flexure of the graduated circles are eliminated in the mean 

 of observations made in the four possible positions of the instrument. 



All these things, and others besides, which it would take too long 

 to enter upon, have been considered in the new instrument now in 

 course of erection. Instead of being mounted in a house with heavy 

 stone walls, the Observatory is a steel structure with vertical walls 

 and a semi-cylindrical roof, the axis of the latter coinciding with the 

 axis of the instrument. The walls and roof are of triple steel, the 

 spaces between the separate y^g-inch thick walls forming ventilating 

 shafts, so that if the sun should heat up the outer sheet a current 

 of air is set up between the walls, and the heated air escapes by 

 ventilating chimneys at the outer ends of the Observatory. 



The two halves of the building are rolled six feet apart by an 

 electro-motor before the instrument is used, so that the instrument 

 stands practically in the open air, and thus all local abnormal effects 

 of refraction are avoided. 



We hope with this instrument to avoid most, if not all, those 

 systematic errors which are the real bane to progress in exact 

 astronomy. 



But there is another class of Star Catalogue which is most neces- 

 sary and important, not only to serve for the nomenclature of the 

 stars, but for the study of stellar distribution in space, viz., a com- 

 plete Catalogue of all stars to a given order of magnitude, with their 

 approximate positions and magnitudes. 



Argelander and his successor Schonfeld had catalogued in this 

 way all the stars to the 9th magnitude from the North Pole to 

 Declination 20° South. It was most desirable to complete that work 

 in the Southern Hemisphere.' 



The Society is aware how the photographing of the great comet of 

 1882, with Mr. Allis' camera attached to an equatorial and the 

 number of star-images photographed on the same plates, led me to 

 realise that photography was the way to make this catalogue. The 

 work was duly set on foot and the photographs taken by Mr. C. Eay 

 Woods ; the plates were measured by Prof. Kapteyn, of Groningen 

 (for our staff was insufficient to overtake this latter additional labour), 

 and we have now a Catalogue of all the stars down to 9J magnitude 

 and most of those to the 10th magnitude between Declination 19° S. 

 to the South Pole. It is published in three large volumes of the 



