SENSIBILITY OF THE EYE 189 



retina. Now "the planet is very faint; and the in- 

 fluence of its feeble light cannot extend far with any 

 degree of equality. This enables us to see the faintest 

 objects, even when they are only a minute or two 

 removed from it. The satellites are very nearly the 

 dimmest objects that can be seen in the heavens; so 

 that they cannot bear any considerable diminution of 

 their light, by a contrast with a more luminous object, 

 without becoming invisible. If then the sphere of 

 illumination of our new planet be limited to 18" or 

 20", we may fully account for the loss of the satellites 

 when they come within its reach ; for they have very 

 little light to lose, and lose it pretty suddenly." This 

 view of a weak light extinguishing a weaker, though 

 a commonplace now, received a very poetical inter- 

 pretation in a paper written by Herschel three years 

 after. " This increased sensibility," he says, " was such, 

 that if a star of the 3rd magnitude came towards the 

 field of view, I found it necessary to withdraw the eye 

 before its entrance, in order not to injure the delicacy 

 of vision acquired by long continuance in the dark. 

 The transit of large stars, unless where none of the 

 6th or 7th magnitude could be had, has generally 

 been declined in my sweeps, even with the 20-feet 

 telescope. And I remember, that after a considerable 

 sweep with the 40-feet instrument, the appearance of 

 Sirius announced itself, at a great distance, like the 

 dawn of the morning, and came on by degrees, in- 

 creasing in brightness, till this brilliant star at last 

 entered the field of view of the telescope, with all the 

 splendour of the rising sun, and forced me to take the 

 eye from that beautiful sight." To increase this sen- 

 sibility of the eye he was on these occasions in the 



