1 68 HERSCHEL AND HIS WORK 



The planet Mercury did not receive much attention 

 from Herschel; but, slight though his interest in it 

 seems to have been, he could not make it a field of 

 observation without shedding light on things then 

 unknown, and afterwards forgotten. As a transit or 

 passage of the planet over the sun's face was due at 

 Windsor in the early morning of November 9, 1802, 

 and Herschel's "apparatus 1 for viewing the sun was 

 then in the highest perfection," he was on the watch 

 for what might happen. The weather proved as 

 favourable as he could wish, and more than forty dark 

 spots were counted on the sun's disc. A little black 

 pea traversing the disc among dark spots of vastly 

 greater size, it might have been feared, would be lost to 

 view or only seen now and again. On the contrary, 

 the black dot was easily seen during the four hours 

 that remained of its passage across. As the sun rose 

 higher, " the corrugations of the luminous solar surface 

 up to the very edge of the planet " were visible with a 

 10-feet reflector. "When the planet was sufficiently 

 advanced towards the largest opening," or spot, " of the 

 northern zone, he compared the intensity of the black- 

 ness of the two objects ; and found the disk of 

 Mercury considerably darker, and of a more uniform 

 black tint, than the area of the large opening." As 

 it approached the edge of the sun, the whole of 

 its disc was "as sharply defined as possible; there 

 was not the least appearance of any atmospheric 

 ring, or different tinge of light, visible about the 

 planet." As the black dot vanished on leaving the 

 bright body of the sun, there was not the slightest 

 distortion of the sun's limb or in its own figure. The 



1 The mirror of the reflector used on this occasion was made of glass. 



