observed with the Great Achromatic of Fraunhofer. 85 



ble ones have been found, we may say that there are nearly 

 one in 40 double ; but this ratio changes with the brightness of 

 the stars. Flamsteed determined more than 100 years ago, in 

 that part of the heavens where we made our survey, the po- 

 sition of 2374 stars, which, for the most part, were from the first 

 to the sixth magnitude. Among these stars of Flamsteed, 

 which have all been examined by Herschel, 169 of the four 

 first classes have been found double, and more recently 63 

 others have been added. Thus among 2374 stars, from the 

 first to the sixth magnitude, there are 230 stars double, that 

 is to say, out of every eleven stars there is one double. The 

 great catalogue of Piazzi contains in the part of the Heavens 

 under our consideration 5762 stars. In subtracting; those 

 of Flamsteed there remain 3388 fainter in a great measure, 

 among which 134 are double, which is at the rate of one in 

 25. As to the stars which are much fainter, and whose posi- 

 tion has been lately determined, the proportion is nearly one 

 double star in 42. We cannot account for such a variety of 

 ratios on the hypothesis of stars optically double. This di- 

 versity gives us a new proof of our former assertions ; and 

 we can explain why the small stars appear so seldom double, 

 from the difficulty of seeing at an immense distance the satel- 

 lite star, which is often much more faint than the principal one. 

 It is remarkable that among the double stars newly dis- 

 covered there are several whose proper motion is already 

 known. I shall mention here only the bright star y Ceti, 

 composed of a star of the third magnitude, and one of the 

 seventh ; No 42 in Berenice's hair, composed of two stars of 

 the sixth magnitude singularly close, and y Corona?, which, 

 on account of the great proximity between the principal star 

 of the fourth magnitude, and its companion of the seventh, is 

 one of the most difficult to be seen. Our catalogue affords a 

 great number of double stars of this kind, many of which had 

 probably escaped preceding observers by the difficulty of dis- 

 tinguishing them, n Herculis, and y Corona?, may be considered 

 in this respect as the best tests of the perfection of telescopes ; 

 and they may help us to compare the power of these instruments 

 with that of the great telescope of Fraunhofer. I have examin- 

 ed with this telescope the double stars of Herschel, of which 



