THE NEBULA ABOUT 6 ORIONIS. 89 



This examination developed the more prominent features, and 

 enabled me to fix upon a convenient scale for the intended 

 drawing. A system of sweeps was then instituted after the fol- 

 lowing manner. The telescope, being clamped in declination, 

 was carried forward until it preceded the utmost limits of the 

 nebula. It was then fixed in right ascension, and the successive 

 fields carefully examined as they passed in review by the diurnal 

 movement of the heavens. The different degrees of intensity of 

 the light were indicated by numbers. Thus, when the first 

 nebulous appearance reached the middle of the field of view of 

 the telescope, the time was noted by a sidereal chronometer, and 

 the degree of light, representing the faintest perception of light, 

 was recorded as 7. When an increase of Jight was discerni- 

 ble, the time was again noted, and the figure 6, indicating a 

 confirmation, was recorded ; this has been adopted generally for 

 the outline of the nebulous district. 5 shows a yet further in- 

 crease of light. In this way, the different portions included in 

 a single sweep were examined, 1 indicating the strongest light, 

 in the vicinity of the Trapezium. 



When it was judged that the whole nebulosity had passed, the 

 hour-angle and declination circles were read off, the declination- 

 circle changed five minutes, and the examination of another par- 

 allel commenced. The results of these sweeps were then 

 reduced to right ascension and declination by differentiating on 

 6^ Orionis, and, being corrected for convergence, were finally trans- 

 ferred to a chart, and are embodied in the drawings which ac- 

 company this memoir. 



These sweeps were extended from half a degree north of the 

 star C Orionis to one degree south of the star t, 0' being con- 

 stantly referred to as the point of departure. 



12 



