ASTRONOMICAL OBSERVATORIES. 255 



fore, by the observer, is first to fix the telescope firmly at 

 any required altitude, and then applying his eye to the s 

 telescope, he observes each star in succession as it enters 

 the field of view. One night's work embraces a zone ten 

 minutes in breadth, and having a length corresponding to 

 the number of hours for which the observations are con- 

 tinued. 



The Messrs. Bond design to include in their series all 

 stars to the eleventh magnitude, and as many of the twelfth 

 as can be got without interfering with the determination 

 of the brighter ones. Each zone is observed a second 

 time as soon as possible, in order not to miss any chance 

 of finding a planet which might happen to be in the way ; 

 but as the object is not planet-hunting, this is not allowed 

 to interfere with the work, further than that missing stars 

 are noted and looked after. It seldom happens that 

 the disappearance does not prove to have originated in 

 some mistake. At the re-observations, all the particulars 

 of the first observation undergo a revision, the original 

 notes being read and compared as the star passes the 

 field. All the double stars, nebulae, remarkable groups, 

 vacancies, etc., are recorded. The comparison of the 

 places of stars which have been previously observed at 

 other observatories forms part of the reduction ; and in 

 this way they have detected some cases of considerable 

 proper motion. 



The Messrs. Bond have completed two entire zones 

 (each twice observed) for the whole circuit of the heavens, 

 from the equator to '20 minutes of north declination. 



