THE NEBULA IN ANDROMEDA. 81 



of this nebula, it was at first proposed to prepare, from micrometric 

 measurements, a map of the principal stars involved in the light. 

 But their great number,* and the consequent danger of confusion, 

 having rendered this impracticable, the circle readings of the equa- 

 torial were resorted to for the determination of all the positions 

 referred to in this memoir. 



The extent of the region to be examined being from fifteen to 

 twenty times larger than could be included within a single field 

 of the telescope, the brightest portions, namely, from Dec. 

 39° 40'' to Dec. 41° 10', were divided into eighteen sections, 

 each comprising five minutes of declination, and extending in 

 right ascension across the nebula. The telescope was clamped 

 in declination at the middle of each zone, and the examination 

 was commenced by moving the instrument with a quick motion 

 in right ascension, which was found necessary in order to deter- 

 mine with any certainty the limit of light on either side of the 

 axis. The nebula was then allowed to pass through the field by 

 its diurnal motion, and the times recorded when the different gra- 

 dations of light occupied the centre of the field, taking in each 

 transit, for the standard of brilliancy, that portion of the axis in- 

 tersected by the zone ; the hour-circle was then read off, and the 

 instrument set for a new series. The different zones were finally 

 referred to a common unit of brightness, by a cross section from 

 the nucleus to both extremities of the axis. In this manner an 

 idea, though not a very accurate one, was obtained of the sit- 

 uation of the lines of equal brilliancy. The observations were 

 then charted, so as to present them at a single view, accom- 

 panied by such remarks as had been recorded at the time at 



* It is estimated that above fifteen hundred stars are visible with the full aper- 

 ture of the object-glass within the limits of the nebula. 



11 



