Engelmann on Double Stars. 139 



uninistakeable black division between the two minute discs of 

 A and B, though, according to Mr. Dawes, their central 

 distance was then only 0"-63. Steinheil has asserted, in expla- 

 nation of this defect in separating power, that the magnitude 

 of the spurious discs of stars depends not only upon the 

 intensity of their light, but also upon the proportion of aper- 

 ture to focal length, which is small in the Leipzig telescope. 

 Airy, however, has shown that, from the undulatory theory of 

 light, this magnitude must depend solely upon amount of 

 aperture, without reference to focal length; and Dawes, 

 speaking from an experience admitting of no appeal, affirms 

 that practice is in accordance with theory. 



8. 7 Virginia. 4"-03. 348°'27 (1864-48). The latter mea- 

 sure is, perhaps, he observes, too large. He also states that 

 both stars appear somewhat variable ; and hence results the 

 difference of his position-angle from those we formerly gave. 

 These will be brought into harmony with it by adding 180°, 

 which is equivalent to reversing the order of brightness ; the 

 more brilliant component being always chosen as the centre of 

 measurement. Demb. has the same estimate. His data are 

 4"-085. 345°-9 (1863-33). 



15. 51 Librce (alias £ Scorpii, entitled by Engelmann 

 % Librce). He failed in separating A and B, noting an elon- 

 gation towards 15°, June 19; 350°, July 22. Demb., too, 

 could only ivedge it, but with measured angles of much greater 

 consistency. This must be a fine test for the largest class of 

 instruments, though unfortunately at rather a low elevation — 



10°-55' S.D.— For A + - and 0, Engelmann gives 7"'41. 



68°-95 (1864-48).— Demb., 5T-154. 70°-46 (1863-12).— 

 Secchi, 7 /, -503. 70°-47 (1855-54).— Sfcruve, 6"*75. 78°-6 

 (1825-48). There is evidently angular motion in this star. 



20. £ Bootis. 5 // -32. 303-°4 (1864-46).— Demb., 5"-59. 

 303°-03 (1863-15).— Secchi, 6"-017. 310°-05 (1856-88). 

 Hence its rapid motion is evident both in angle and distance. 



22. f Bootis. 0"-87. 306° (1864-5).— Secchi, 0"-995. 

 305°-7 (1855-7).— Struve, 1"-189. 309°-17 (1830-47). 

 Here we detect, at last, pretty certain evidence of motion in a 

 double star, formerly considered stationary. By drawing up 

 closer, while it gains the character of an unquestioned binary, 

 it loses some value as a permanent test-object, for which pur- 

 pose, however, it answers as yet admirably. My 5^-inch 

 aperture divides it at present well with a power of 450, giving 

 a dark interval equal to about 0*2 of either disc. Its gradual 

 approach should be watched by all observers whose instruments 

 can reach it, and measured by all whose apparatus can master 

 it. It is not improbable that this £*, and £ Cancri already 



