204 



STELLAR PIIOTOGRAPHY. 



ing the telescope when covering or uncovering it, as the slightest flexure is suffi- 

 cient to distort the ends of the trails. The measures are necessarily relative except 

 in the case of polar stars, whose absolute declinations may be determined if the 

 centre of the circles constituting the trails can be fixed with sufficient precision. 



Unfortunately, the method of trails is not applicable to very faint stars unless 

 they are near the pole. A close polar star no brighter than the fourteenth magni- 

 tude gives a satisfactory trail, but equatorial stars fainter than the eighth magnitude 

 have not as yet been photographed in this way, on account of their rapid motion. 



As stated above, short trails are produced whenever the telescope is not perfectly 

 adjusted. This is an objection to the appearance of the images upon a chart, and 

 prevents the faintest stars from forming images. On the other hand, the advantages 

 of distinguishing the images from defects in the plate, and the greater accuracy with 

 which the brightness can be measured, may render it advisable to employ this 

 method. 



A wide field of work appears open in the application of photography to meridian 

 instruments, or to the almucantar. The sensitive plate should be substituted for 

 the reticule, and the position marked by a graver attached to the tail-piece of the 

 telescope. The times may be indicated by covering and uncovering the object- 

 glass automatically by the sidereal clock. The intervals should be determined by 

 trial, so as to give a series of nearly circular dots separated by as short intervals 



possible. Another method is to attach the plate to the armature of an electro- 

 magnet, the current being made and broken at regular intervals. Two series of 



as 



alternate lines of dots are thus formed. A large number of stars may be recorded 

 on a single plate. The principal advantage of this method would be its freedom 

 from personal equation. It is therefore especially adapted to longitude campaigns. 

 Moreover, a high degree of skill is not required by the observer. It would not be 

 necessary to employ a large telescope in this work. A 3-inch object-glass with a 

 focal length of 44 inches would give a sixth-magnitude star as well as an eighth- 

 magnitude star is shown in the Bache telescope. 



A large number of photographs have been taken of the immediate vicinity of 

 the pole, both with and without clockwork. A special section of this memoir is 

 devoted to the discussion of a portion of them. (See page 218.) 



The second research to be described is undertaken to determine the light of all 

 of the brighter stars. In order to include all portions of the sky, several expos- 

 ures are made on each plate. The region to which each star belongs is indicated 

 by varying the time of exposure. Since the length of the trail is proportional 



