430 



NATURE 



[August 30 1900 



van' thus corresponds with the angle Y in Prof. Cornu's formula, 

 and its value is derived by precisely the same formula. 



Fig. 3.— Orientation of field of siderostat. 



The orientation of the field of a coelostat is very readily 

 derived. In this instrument the mirror turns on a polar axis 

 in its own plane, so that the normal is 

 always on the equator, and the polar dis- 

 tance of the reflected ray is always equal 

 to the supplement of the polar distance 

 of the star. Thus, in Fig. 4, pd' is the 

 supplement of pd. The reflection of the 

 hour circle through the star, pd, will coin- 

 cide in direction with that through the re- 

 flected ray, pd', so that n will become «', 

 and it only remains to determine the angle 

 pd'z to ascertain the position of the north 

 point with regard to a vertical line through 

 the field of the mirror. If we suppose the 

 rays to be reflected in a horizontal direction, 

 in the triangle pzd, pz = the co-latitude, 

 ; zd' = 90° and pd' = 180° - pd, so that the 

 required angle can be at once derived. In 

 this case it is convenient to know the 

 azimuth of the reflected ray, that is, pzd' ; 

 and the simplest solution is to calculate 

 this angle first by the formula 



cos pzd' = cos (180° -pd) sec L. 



The required angle is then derived from 

 the formula 



sin pd'z = sin pzd' cos L cosec (180° - pd). 



The position of the north point having 



Jaeen determined, the remaining points can 



at once be placed, noting that the east and 



west points are reversed as compared with 



the direct view in the sky. 



It is important to note that although there is no rotation of 

 the field so long as the telescope remains in one position, the 



tions of the morning sun, the orientation of the field will be 

 different when the telescope is pointed in an easterly direction 

 for observations of the sun in the afternoon ; in the former case 

 the north point lies to the right of the vertical, and in the latter 

 case to the left. 



Other investigations relating to the coelostat, including the 

 determination of the best position for the telescope under given 

 conditions, have been made by Prof. H. H. Turner {Monthly 

 Notices R.A.S., vol. Ivi. p. 408). 



As the coelostat has not yet come into very general use, it 

 may be of interest to add a few remarks as to the arrangements 

 which have been made by Sir Norman Lockyer at the Solar 

 Physics Observatory for utilising this instrument in a permanent 

 observatory (Fig. 5). On account of the varying declinations 

 of the heavenly bodies, the position of the observing telescope 

 must admit of corresponding changes, either in inclination or 

 azimuth, or both. When special instruments, such as the 

 spectro-heliograph, are to be used with the coelostat, as at the 

 Solar Physics Observatory, motion in azimuth is the only motion 

 permissible, and this is provided for by fixing the receiving 

 instrument on a platform which runs on circular rails, with the 

 coelostat at the centre. The platform carrying the telescope or 

 spectroscope is covered with a travelling hut, the roof of which 



Fig. 4. — Orientation of field of coelostat. 



whole field is turned when the telescope is set in a different 

 direction. Thus, if the telescope is directed west for observa- 



Observatory. 



is inclined so as not to obstruct the mirror. The coelostat itselr 

 is provided with a hut, which is removed to the north when the 

 instrument is in use ; this is shown to the. left in the illustration. 



A. Fowler. 



A 



THE ANNIVERSARY MEETING OF THE 

 RE ALE ACCADEMIA DEI LINCEI. 

 MELANCHOLY interest attaches to the anniversary 

 meeting of that ancient scientific society, the Reale Ac- 

 cademia dei Lincei, held in June, from the fact that the society 

 was then mourning the loss of its distinguished president, Prof. 

 Beltrami, and has since been plunged into deeper mourning by 

 the untimely and unexpected loss of its patron, King Humbert, 

 who with Queen Margherita had for many years taken part in 

 these yearly meetings. It is, moreover, largely due to the 

 munificence of the late King of Italy that the society is enabled 

 to further the advancement of science by the award of prizes for 

 theses dealing with some subject of scientific research. 



From the presidential report of Prof. Mesadaglia, we learn 

 that the society's losses have included, besides Beltrami, the 

 names of Capasso, De Simoni, Ferrara, Nestore and Tommasi- 

 Crudeli among the ordinary members, and, of foreign members. 

 Bertrand, Bunsen, Janet and Liais. The Atti, or " Proceedings " 

 contain for the year 147 papers, in addition to which several 



NO. 1609, VOL. 62] 



