VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. 643 



that collection for an explanation of several particulars necessary to be previously 

 known. The numbers of the stars are here also continued, so that the first 

 class ending there at 24 begins here at 25, and the same is done with the other 

 classes. 



Most of the double stars in my first collection are among the number of those 

 stars which have their places determined in Mr. Flamsteed's extensive catalogue; 

 but of this collection many are not contained in that author's work. I have 

 therefore adopted a method of pointing them out, which it will be proper to 

 describe. The finder of my reflector is limited, by a proper diaphragm, to a 

 natural field of 2° of a great circle in diameter. The intersection of the cross 

 wires, in the centre of it, points out 1°; and by the eye this degree, or the dis- 

 tance from the centre to the circumference, may be divided into ^, -l, $, J-, and 

 -§.. Thus we are furnished with a measure which, though coarse, is however 

 sufficiently accurate for the purpose here intended ; and which, if more than 2° 

 are wanted, may be repeated at pleasure. In such measures as these I have 

 given the distance of a double star, whose place I wanted to point out, from the 

 nearest star in Flamsteed's catalogue. And since, besides the distance, it is also 

 required to have its position with regard to the star thus referred to, I have used 

 the neighbouring stars for the purpose of pointing it out. 



It will sometimes happen, that other stars are very near those which are thus 

 pointed out, that might be mistaken for them. In such cases an additional pre- 

 caution has been used by mentioning some circumstance either of magnitude or 

 situation, to distinguish the intended star from the rest. After all, if any ob- 

 server should be still at a loss to find these stars without having their right ascen- 

 sion and declination, he may furnish himself with them by means of Flamsteed's 

 Atlas Ccelestis ; for my description will be sufficiently exact for him to make a 

 point in the maps to denote the star's place; then, by means of the graduated 

 margin, he will have its ar and declination to the time of the Atlas, which he 

 may reduce to any other period by the usual computations. 



Before quitting this subject I must remark, that it will be found on trial, that 

 this method of pointing out a double star is not only equal, but indeed superior, 

 to having its right ascension and declination given : for, since it is to be viewed 

 with very high powers, not such as fixed instruments are generally furnished 

 with, the given right ascension and declination would be of no service. We 

 might, indeed, find the star by a fixed or equatorial instrument ; and, taking 

 notice of its situation with regard to other neighbouring stars, find, and view it 

 afterwards, by a more powerful telescope ; but this will nearly amount to the 

 very same way which here is pursued, with more deliberate accuracy than we are 

 apt to use, while we are employed in seeking out an object to look at. 



It will be required, that the observer should be furnished with Flamsteed's 



4 N 2 



