THE PLANET NEPTUNE. 21 



with undertaken. The observatory of Cambridge is 

 provided with one of the finest telescopes of Europe, 

 presented by the late Duke of Northumberland. Pro- 

 fessor Airy urged upon the director, Professor Challis, 

 to undertake the desired search; and recommended the 

 examination of a belt of the heavens ten degrees in 

 breadth, and extending thirty degrees in the direction of 

 the ecliptic. This belt was to be swept over at least 

 three times. If any star in the second sweep had a dif- 

 ferent position from that observed in the first, it might 

 be presumed that it was the planet. If two sweeps 

 failed of detecting the planet, it might be caught in the 

 third. 



Professor Challis commenced his search July 29th, and 

 continued it each favorable evening, recording the exact 

 position of every star down to the eleventh magnitude. 

 Meanwhile Le Yerrier was proceeding with his computa- 

 tions, and on the 31st of August he announced to the 

 Academy the elements he had obtained for the sup- 

 posed planet. He assigned its exact place in the heav- 

 ens, and estimated that it should appear as a star of 

 the eighth magnitude, with an apparent diameter of 

 about three seconds ; and, consequently, that the planet 

 ought to be visible in good telescopes, and with a per- 

 ceptible disc. The fixed stars are situated at such im- 

 mense distances from us, that to the most powerful 

 telescopes they appear only as points, although with a 

 brilliancy augmented in proportion to the size of the tel- 

 escope; while all the planets exhibit a measurable disc. 



