l64 PHILOSOPHICAL TRANSACTIONS. [aNNO 1786. 



greatly add to the reception of this opinion when the telescope exposed to our 

 view Jupiter, Mars, and Venus, revolving on their axes;* and if these instances 

 of the similar condition of other planets support the doctrine of the diurnal mo- 

 tion, the view of so many sidereal systems, some of which we may discern to be 

 of a most surprising extent and grandeur, will in like manner add credit to what 

 I have proposed with regard to the condition of our situation within a system of 

 stars; for, to the inhabitants of the nebulae of the present catalogue, our side- 

 real system must appear either as a small nebulous patch; an extended streak of 

 milky light; a large resolvable nebula; a very compressed cluster of minute stars 

 hardly discernible; or as an immense collection of large scattered stars of various 

 sizes. And either of these appearances will take place with them according as 

 their own situation is more or less remote from ours. 



In the distribution of the nebulae and clusters of stars into classes, I have 

 partly considered the convenience of other observers : thus, in the first class, the 

 degree of brightness of the nebulae has been the leading feature, as most likely 

 to point out those which their several instruments may give them expectation to 

 reach. The 1st class therefore contains the brightest of them; the 2d, those 

 that shine but with a feeble light; and in the 3d are placed all the very faint ones. 

 Besides this general division, I have added a 4th and a 5th class, which contain 

 nebulae that, on different accounts, seemed to deserve a more particular descrip- 

 tion than I had allotted to the 3 former divisions. The clusters of stars are 

 sorted by their apparent compression, in the manner of my former catalogues of 

 double, treble, and multiple stars; so that the closest and richest clusters take 

 up the 1st class; the brightest, largest, and pretty nmch compressed ones, the 

 2d ; and those which consist only of scattered and less collected large stars, are 

 put into the last. 



In every class, the order of time when the nebulae and clusters of stars were 

 discovered, or first observed with my 20-feet telescope, has been followed; and 

 that I might describe all these objects in as small a compass as could well be done, 

 I have used single letters to express whole words, an explanation of which, with 

 an example of the manner of reading those letters, is given. It should be ob- 

 , served, that all estimations of brightness and size must be referred to the instru- 

 ment with which the nebulae and clusters of stars were seen; the clearness and 

 transparency of the atmosphere, the degree of attention, and many more parti- 

 cular circumstances, should also be taken into consideration ; so that probably 

 some of the nebulae which I have called very bright, and very large, may only 

 be just perceivable, as very small faint patches, in many of our. best common 

 telescopes. 



» To these may now also be added Saturn, on whose body I have, in the year 1780, seen several 

 beltSj with spots that changed their situation in the course of a few nights. — Orig. 



