90 THE NEBULA ABOUT 6 ORIONIS. 



This general method, however, would not answer for delineat- 

 ing the more delicate and intricate portions ; for this purpose, 

 as well as for the accurate location of the stars, it became 

 necessary to have recourse to the micrometer. With this instru- 

 ment, such stars as appeared to be favorably situated were ar- 

 ranged in groups by an eye sketch. The star 0' being adopted 

 in the first instance as a primary station, differences of right as- 

 cension and declination, or of position and distance, were meas- 

 ured : when the distances became excessive, new stations were 

 occupied, taking care always to preserve the connection with the 

 preceding point by repeated measures. 



A catalogue of the stars thus differentiated accompanies • the 

 memoir. No attempt has been made to locate every star that 

 was visible within the boundaries of the nebula, for the reason 

 that no apparent advantage, at all adequate to the great expen- 

 diture of time and labor which it would have required, was to 

 be expected. 



In order to obtain a correct outline of the more important 

 points in the figure of the nebula, the stars contained in the 

 annexed catalogue were, in the first instance, laid down accord- 

 ing to their observed differences of right ascension and declina- 

 tion. All such parts of the nebula in the vicinity of the Trape- 

 zium as presented definite outlines susceptible of being measured 

 were referred to 6^. Guided by these points, the outline was 

 drawn and filled in, after many repeated examinations of the 

 object under different powers. 



In the course of these examinations with different eye-pieces, 

 I was struck with a remarkable diversity in the appearance of 

 the Huygenian region. It seems, as we increase the power of 

 our eye-piece, that the clouds or clusters into which this region 



