84 Professor Struve on Double and Triple Stars, as 



those of the third class. The ratio of the number of double 

 stars found in the various classes thus supplies us with a cri- 

 terion for knowing the stars which are physically double. It 

 is evident from what we have said, that the determination of 

 the parallax of the fixed stars, according to Herschers method, 

 could not, as was before supposed, be attempted with any hope 

 of success ; for this method being only possible for stars opti- 

 cally double, could never consequently be applied to double 

 stars of the first or second class, but solely to those of the 

 fourth, and beyond it. The bright double stars, that is to 

 say, those where the two^tars of the first to the seventh mag- 

 nitude appear joined together, deserve particular consideration. 

 The new catalogue contains 207, among which there are but 

 69 new ones, and this is not surprising, as preceding ob- 

 servers had principally turned their attention to these stars. 

 Among the bright double stars, those of the first class are the 

 most numerous, for there are 92 of that class, 33 of the second, 

 34 of the third, and 48 of the fourth. In the 69 new stars 

 there are 49 of the first class, which shows the extraordinary 

 power of our telescope ; for Herschel examined a great many 

 without finding out that they were double stars of that class. 

 We easily perceive that all the double bright stars, even those 

 of the fourth class, ought to be considered as physically double. 

 I have obtained a confirmation of this fact in the following man- 

 ner. 



In the celestial maps of Harding, which may be considered 

 as perfect, as far as regards stars of the seventh magnitude, 

 we reckon 10229 stars of the first to the seventh magnitude, 

 even to the distance of fifteen degrees south of the equator. 

 If we apply to this number the calculation of probabilities, we 

 shall obtain the following very remarkable result. It is, that 

 we ought to find in this space but one pair of stars, thirty-two 

 seconds distant from one another. If then it is possible that 

 some one of the double bright stars of the third and fourth 

 classes are in a manner optically double, all the double stars of 

 the first class, and a great part of those of the last, ought to be 

 considered physically double, or as forming a particular sys- 

 tem of two stars joined together. 



Since among 200,000 observed stars more than 3000 dou- 



