Observations of 500 Nebula. 9 



Before terminating this introduction it appears desirable, with respect 

 to the conditions necessary for the obtaining of a trustworthy nebular po- 

 sition, to add the following. First it ought to be mentioned, that in my 

 series of observations some cases occur, in which the discrepancies between 

 the various positions are very considerable, although the character of the 

 objects does not appear to be such as to lead us to expect anything of the 

 kind. The true explanation of these discrepancies, whether they are real 

 errors of observation or not, can only be obtained by means of new obser- 

 vations; but the possibility of committing errors of observation to this 

 amount does not appear to me a priori inexplicable, but needs not neverthe- 

 less preclude the possibility in general of determining the positions with 

 great accuracy. It is in fact well known, that the rareness of the occasions, 

 on which nebulae can be advantageously observed, causes the work during 

 the few hours available for the purpose to be too frequently prosecuted with 

 a sort of anxious haste , which may easily account for errors of various kind 

 creeping into the observations. When experience had taught me, that ne- 

 bula-observations thus hurriedly made could only have a very ambiguous 

 value, I never afterwards attempted to execute an observation in too scanty 

 a time, but in the earlier period of my observations the contrary Avas not 

 so unfrequently the case. Such discordant positions can even arise from 

 a partially false perception of the objects on occasions very disadvantageous 

 for observation, when, according to my present opinion, uebulai ought not 

 to be observed at all. Bad atmospheric condition may in fact utterly dis- 

 figure the nebula and thus often give the observer an intirely false idea of 

 it. But even, when the atmosphere is unexceptionable, an observer may 

 easily commit a sensible error of pointage, unless he give himself suffici- 

 ently time; for it often happens, that the point, which for objective reasons 

 ought to be selected as that, to which the instrument should be directed, 

 is on close inspection pretty conspicuous, but would not strike the eye in 

 a hasty and passing view. In order that a nebula's position may conse- 

 quently be reliably determined, it is in my opinion generally speaking in- 

 dispensably necessary, that the observations should be made with all due 

 calmness and in a sufficiently favourable atmosphere. It is moreover per- 

 haps not too much to assume, that the satisfactory carrying out of nebula- 

 observations requires more custom and more practice than those of many 

 other objects. 



It belongs lastly, as I have already intimated, to the conditions ne- 

 cessary for the obtaining of a trustworthy nebular position, that the instru- 

 mental means are fitted for the purpose, that is generally speaking to say, 



Nova Acta Reg. Soc. Sc. Ups. Ser. III. 2 



