Clusters and Nebulce. 463 



of collected facts. And where movement is imperceptibly 

 tardy, observation can but wait upon it with, corresponding 

 patience. In these matters we have reason to believe, though 

 not to be absolutely confident, that motion is all but imper- 

 ceptible; but we must recollect what has happened to the 

 primitive belief as to the immobility of the so-called Fixed 

 Stars. Change of place in the collective body, which has 

 already been suspected by great observers, and which would 

 be in harmony with the " proper motions" of unnumbered 

 solitary or binary stars, could only be dealt with in the 

 majority of cases by appropriate methods of measurement ; the 

 graduated instruments of observatories would, of course, be 

 always applicable, but perhaps seldom necessary, if Alvan 

 Clark's ingenious, beautiful, and far less costly micrometer for 

 measuring large distances (Monthly Notices, xix. 324) were 

 brought into use. The range of this apparatus being about a 

 degree, the cases would not be many in which a cluster could 

 not be compared, both in position and distance, with several 

 surrounding stars ; and though such comparisons would be 

 singly of little weight, their precariousness would disappear 

 under a multitude of repetitions, while the employment, where 

 practicable, of several stars in different bearings, would detect 

 any material error arising from proper motion amongst them. 

 But besides this, each cluster possessing sufficiently salient 

 points should be watched for a much more interesting phe- 

 nomenon — internal change. This is so far less probable than 

 spatial movement, as it is unsupported by any other except the 

 most general analogy ; but what, of such things, ought to be 

 pronounced impossible ? Some great authorities would not so 

 pronounce it. And some clusters are well enough marked to 

 show it, by the distinctness and individuality, or marked 

 arrangement of their brighter components. 



Another noticeable feature in stellar clusters would be 

 difference of colour in different parts. Such a variation is not 

 without an analogy, which, however slight and distant, should 

 no more be neglected in an investigation where we have so 

 little to aid us, than some faint foot-print would be disregarded 

 by the traveller in a pathless desert. Notwithstanding what 

 may at first appear the fortuitous dispersion of colour among 

 insulated stars, I have been led to notice so striking a pre- 

 valence of an uniform, tint in some regions, as to believe that a 

 general and careful review of the whole heavens, with regard 

 to this point, would be desirable. Not only would this be 

 interesting in regard to possible change, but it might lead to 

 some result as to the mode of distribution ; and I have been 

 gratified by observing that the idea is corroborated by the 

 spectrometric researches of Secchi, who has detected a preva- 



