132 CARNEGIE INSTITUTION 



\jFrom Dr. Ralph Copelayid, Astrono^ncr Royal for Scotland. '\ 



Edinburgh, 26th August, igoj. 



I have read your letter and inclosure of Jul}^ 8 with deep interest. 

 Regarding the various classes of special observations to be made in 

 the southern hemisphere, included in your statement, it seems to 

 me that classes i, 4, and 3 are of the greatest immediate importance, 

 and their urgency is probably in the above sequence, the first being 

 the most important. 



Class 2 I consider of relatively far less immediate interest, seeing 

 that the results arrived at are by no means of the fundamental 

 character of those obtained by classes i, 4, and 3. 



Respecting class i , it is conceivable that the observations might 

 advantageously be divided into two groups : a, the fundamental 

 determinations of the places of 600 principal stars, together with 

 the essential observations of the sun ; d, the precise observation of 

 the 5,400 remaining stars brighter than the seventh magnitude be- 

 tween — 20° and the south pole. 



Group a. — Possibly this work could be most satisfactorily accom- 

 plished by using two instruments — a vertical circle and a specially 

 efficient transit instrument. But these instruments, and in particu- 

 lar the cells of the object glasses, should be made of mild steel, 

 which has a coefficient of expansion dififering but little from that of 

 glass and much smaller than that of brass, hitherto so largely used 

 in the construction of astronomical instruments of precision. With 

 the lenses held by springs on the plan designed by Fraunhofer, it is 

 probable that they would rest almost absolutel}^ immovable in their 

 cells in all positions of the instrument, and that their minute real 

 movements would be directly related to changes of temperature. 



For the vertical circle I would suggest the trial of a design that 

 occurred simultaneously to the late Dr. Common and to myself. It 

 consists in placing an object glass at each end of the tube in such 

 manner that the focus of either object glass shall fall absolutely on 

 the outer surface of the other. Spiders' webs are to be replaced by 

 fine lines, engraved or etched on the outer surface of each object- 

 glass (I have seen lines of this kind not appreciably inferior to the 

 finest natural webs) . This construction would permit of the ' ' end 

 for end" reversal of the vertical circle, as well as of the ordinary 

 " right and left " reversal, thus eliminating flexure from every de- 

 termination of double zenith distance. I pass over the obvious 



