134 Mary Somerville. 



also constituents of the sun, the stars, and even of 

 the nebulae. Dr. Wollaston gave me the little prism, 

 which is doubly valuable, being of glass manufac- 

 tured at Munich by Fraunhofer, whose table of dark 

 lines has now become the standard of comparison in 

 that marvellous science, the work of many illustrious 

 men, brought to perfection by Bunsen and Kirchhoff. 

 * * * * 



Sir William Herschel had discovered that what 

 appeared to be single stars were frequently two stars 

 in such close approximation that it required a very 

 high telescopic power to see them separately, and 

 that in many of these one star was revolving in an 

 orbit round the other. Sir James South estab- 

 lished an observatory at Camden Hill, near Kensing- 

 ton, where he and Sir John Herschel united in 

 observing the double stars and binary systems with 

 the view of affording further data for improving our 

 knowledge of their movements. In each two observa- 

 tions are requisite, namely, the distance between the 

 two stars, and the angle of position, that is, the angle 

 which the meridian or a parallel to the equator 

 makes with the lines joining the two stars. These 

 observations were made by adjusting a micrometer 

 to a very powerful telescope, and were data suffi- 

 cient for the determination of the orbit of the 

 revolving star, should it be a binary system. I have 



