1891.] on An Astronomer's Work in a Modern Observatory. 407 



I will not speak now of tliese researclies, because they are still in 

 progress of execution or of reduction. I would rather, in the first 

 place, endeavour to complete the picture of a night's work in a 

 modern observatory. 



We pass on to celestial photography, where astrometry and astro- 

 physics join hands. Here on the screen is the interior of one of the 

 new photographic observatories, that at Paris. [Brief description.] 



Here is the exterior of our new photographic observatory at the 

 Cape. Here is the interior of it, and the instrument. [Brief 

 description.] 



The observer's work during the exposure is simply to direct the 

 telescope to the required part of the sky, and then the clockwork 

 nearly does the rest — but not quite so. The observer holds in his 

 hand a little electrical switch with two keys ; by pressing one key he 

 can accelerate the velocity of the driving screw by about 1 per cent., 

 and by pressing the other he can retard it 1 per cent. In this way he 

 keeps one of the stars in the field always perfectly bisected by the 

 cross wires of his guiding telescope, and thus corrects the small 

 errors produced partly by changes of refraction, partly by small 

 unavoidable errors in cutting the teeth of the arc into which the 

 screw of the driving shaft of the clockwork gears. 



The work is monotonous rather than fatiguing, and the com- 

 panionship of a pipe or cigar is very helpful during long exposures. 

 A man can go on for a watch of four or five hours very well, taking 

 plate after plate, exposing each, it may be, forty minutes or an hour. 

 If the night is fine a second observer follows the first, and so the 

 work goes on the greater part of the night. Next day he develops 

 his plate and gets something like this. [Star cluster.] 



Working just in this way, but with the more humble apparatus 

 which you see imperfectly in the picture now on the screen, we have 

 with a rapid rectilinear lens by Dallmeyer of 6 inches aperture 

 photographed at the Cape during the past six years the whole of the 

 southern hemisphere from 20° of south declination to the south pole. 



The plates are being measured by Professor Kapteyn, of Groningen, 

 and I expect that in the course of a year the whole work containing 

 all the stars to 9J magnitude (between 200,000 and 300,000 stars) 

 in that region will be ready for publication. This work is essential 

 as a preliminary step for the execution in the southern hemisphere of 

 the great work inaugurated by the Astrophotographic Congress at 

 Paris in 1887, the last details of which were settled at our meeting 

 at Paris in April last. What we shall do with the new apparatus 

 perhaps I may have the honour to describe to you some years hence, 

 after the work has been done. 



We now come to an important class of astronomical work more 

 purely astrophysical, for the illustration of which I can no longer 

 appeal to the Cape, because I regret to say that we are not yet pro- 

 vided with the means for its prosecution. I refer to the use of the 

 spectroscope in astronomy, and especially to the latest developments 



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