Observations of 500 Nebulae. 



13 



The observed differential coordinates, as has already been intimated, 

 consist of differences of right ascension observed with chronometer with eye 

 and ear, and of differences of declination measured with the micrometer- 

 screw, in which illumination of the wires in dark field always was employed. 

 On many occasions it would have been desirable to determine angles of 

 position and distances, but through the want of a good driving-clock such 

 measures in general were frustrated; and this want was often in the nebula- 

 observations even more sensible than in those of the star-clusters at least 

 of CI. VII and CI. VIII. In the first years of observation I indeed excep- 

 tionally tried to measure distances, but under the existing circumstances 

 always found this operation too difficult and unsatisfactory to be repeated. 

 Angles of position, as well as it could be done, I have nevertheless on 

 several occasions measured with better success; but in the last years even 

 those measures have been generally avoided and consequently only executed, 

 when the circumstances were more favourable and such measures to be re- 

 garded as quite indispensable. The dimensions of the few nebulae, which 

 on favourable occasions could be measured with passable exactness — as for 

 instance the planetary — were also for the said reason only determined in 

 the directions of the parallel and declination-circle. For the determination of 

 the positions of the axes of elongated nebulae I have, as often as a rela- 

 tively exact measure was possible and the time permitted it, observed their 

 angles of position, which of course, even without a driving-clock, could be 

 done with an accuracy quite sufficient for the purpose. The results of those 

 and "the before mentioned measures of the sizes of some nebulas I give in 

 note II below. Concerning the observations in general it ought here to be 

 remarked, that in almost all cases a magnifying power of 210 was employed, 

 wherefore it is not necessary to indicate the few and in general insignificant 

 exceptions from that rule; some such cases are however mentioned in note II. 

 As my eye generally requires strong illumination , 320 m. p. in most cases 

 made the objects too faint, but with 160 m. p. the details of the nebulae 

 on the other hand did not come out with the precision , which by the use 

 of 210 m. p. was guarantied without any serious sacrifice of brightness. 



The differential coordinates are by the application of all astronomical 

 corrections first reduced to the mean equinox for the commencement of the 

 year of observation, although the numbers of reduction — with exception of 

 those for refraction in some cases — were usually exceedingly small; and 

 the results so obtained then by means of precession reduced to the mean 

 equinox of 1865,0. 



