NOTES TO BOOK VII. 313 



versality of the rule. Perhaps the minds of astronomers 

 are still in suspense upon the subject. But I believe I 

 may venture to say that when Sir John Herschel's work 

 shall appear, it will be found that with regard to some 

 of these stars, and y Virginis in particular, the con- 

 formity of the observations with the laws of elliptical 

 motion amounts to a degree of exactness which must 

 give astronomers an irresistible conviction of the truth 

 of the law. For since Sir W. HerscheFs first measures 

 in 1781, the arc described by one star about the other 

 is above 305 degrees ; and during this period the angular 

 annual motion has been very various, passing through 

 all gradations from about 20 minutes to 80 degrees. 

 Yet in the whole of this change, the two curves con- 

 structed, the one from the observations, the other from 

 the elliptical elements, for the purpose of comparison, 

 having a total ordinate of 305 parts, do not, in any part 

 of their course, deviate from each other so much as two 

 such parts. 



(u.) p. 273. Littrow, in his Die Wunder des Him- 

 mels, Ed. 2. pp. 684, 685, says that Gascoigne invented 

 and used the telescope with wires in the common focus 

 of the lenses in 1640. He refers to Phil. Trans, xxx. 

 603. Picard reinvented this arrangement in 1667. I 

 have already spoken of Gascoigne as the inventor of the 

 micrometer. 



Homer, (already mentioned, p. 263) brought into use 

 the Transit Instrument, and the employment of complete 

 Circles instead of the Quadrants used till then, and by 

 these means gave to practical astronomy a new form, 

 of which the full value was not discovered till long after- 

 wards. Littrow s Note. 



