Observations of 500 Nebulae. 



5 



entirely unfit for these observations, which can not be advantageously made 

 except in a dark and clear sky. It should be further remarked that the 

 aurora is in many instances as formidable an enemy to these observations 

 as moderate moonlight, since on such occasions, not to mention the illumi- 

 nation of the sky, the air, even when the aurora is but faint, often becomes 

 very opake, just as if the heavens were curtained with a tine veil of cirrhi. 

 All these circumstances combined render it impossible to follow out in detail 

 any determinate plan, so that long periods often have intervened before a 

 control-observation could be obtained, and in some cases indeed this confir- 

 mation is still wanting. In many cases for example I have had to look up 

 the same nebula, not belonging to the brighter class, as often as five or 

 six times before I was fortunate enough to get an evening on which I could 

 see and observe it satisfactorily. 



Besides all this it must, as I have already said, be added, that, even 

 under tolerably favourable circumstances , the work must proceed very slowly, 

 whenever greater accuracy is aimed at and fainter nebulae are observed. 

 I can not on an average assume, that a satisfactory determination of posi- 

 tion can be obtained in less time than an hour, even when a nebula is 

 pretty bright and the star of comparison not too distant. And though in 

 some cases this time may be somewhat curtailed, yet the same operation 

 will require under legs favourable circumstances frequently double that time. 

 In some cases in order to get through an observation and thus obtain 

 something like a position , the determinations have therefore not so unfrequently 

 — especially in the earlier years of the series — been based upon a decidedly 

 insufficient number of individual observations. Moreover if our business be 

 in certain cases to determine a nebula's position with respect to the fainter 

 neighbouring stars, and to study it under favourable atmospheric circumstan- 

 ces in a dark field, it is evident, that a long time must in most instances 

 elapse, before one could be ready with a single nebula. Here is also the 

 place to remark, that the determination of accurate positions of nebula? is 

 a work, that soon tires the eye, so that even during the most favourable 

 evenings it can not be arbitrarily continued for any length of time. On the 

 most favourable occasions I have somtimes been able to continue working 

 uninterruptedly for 6 hours, or even somewhat longer, but in most cases, 

 when the state of the atmosphere was not so excellent, half that time has 

 often been found sufficient to tire the eye, and render further observation 

 impossible. 



What has here been said could seem to lead to the conclusion, that the 

 results obtained can hardly be considered as in any reasonable degree pro- 



