430 An Account of a General Survey of the Heavens 



From the readings made by these microscopes it is easy to de- 

 duce what the immediate observation of the four verniers and the 

 level would have given for each star. For, the place of the point 

 of commencement of the scale of the microscope is sought by 

 selecting an arbitrary point within the circuit of the zone, not 

 only with the microscope, but also on the verniers, &c. With 

 this place is compared the reading for every star : by which 

 means also the declinations may be found, not by the diffe- 

 rence between known stars, but by actual observations. The 

 right ascensions will be given immediately by the fundamental 

 stars, while the position of the instrument, with regard to the 

 meridian, is always most exactly known ; so that the observa- 

 tions of the zones are founded upon the same basis as that 

 upon which the other observations of mine depend. In this 

 sole dependence on all the preceding determinations consists, 

 without doubt, one essential advantage of this new mode of 

 observation, the peculiarities of which 1 will now explain some- 

 what more in detail. 



These observations require two observers, one of wliom 

 must attend to the right ascension and to the placing the mid- 

 dle of the horizontal wire upon the stars, whilst the other ob- 

 serves the microscope ; the former of these is myself, the latter 

 is Dr. Argelander. 



When the survey of a zone is to be made, the hammer is 

 screwed up so as to indicate the boundary of such zone ; and 

 the subsidiary arc is so placed that the middle of it may fall on 

 the middle of the zone: the northei'n and southern boundaries of 

 the zone and the level are then i"ead from the four verniers by the 

 microscope, and the state of the meteorological instruments 

 noted. After these preparations the observer moves the telescope 

 slowly up and down until a star appears in the field of view; 

 the liorizontal wire is placed on it ; and this being marked by 

 a signal to the second observer, the time of transit is observed 

 by means of one wire, the magnitude and other properties* of 

 the star are noted, and each observer writes down his part of 

 the observation ; by which means an arrangement is obtained 

 at once favourable to oeconomy of time, and to security from 

 error. In this manner the observations may be continued (un- 

 less clouds should intervene) for at least an hour and a half 



* Under favourable circumstances the magnifying power of this astonish- 

 ing telescope ( = 107) exhibits double stars, of the first class, at the first 

 glance; more are already discovered, as well as double stars of the re- 

 maining classes. In an unfavourable atmosphere, however, the smaller 

 ones of the first class may easily ])ass unobserved : but the stars are 

 often so indistinct that it is impossible to calculate on any discoveries of 

 tili^ kind. 



wilhoilt 



