S4 JDr. Herschel om. £fo Power of 



that the tranquillity of the retina, which is not disturbed by 

 foreign objects of vision, may render it fit to receive impressions 

 such as otherwise would have been too faint to be perceived. 

 This seems to be supported by telescopic vision ; for it has often 

 happened to me, in a fine winter's evening, when, at midnight, 

 and in the absence of the moon, I have taken sweeps of the hea- 

 vens, of four, five, or six hours duration, that the sensibility of 

 the eye, in consequence of the exclusion of light from surround- 

 ing objects, by means of a black hood which I wear upon these 

 occasions, has been very great ; and it is evident, that the open- 

 ing of tfie iris would have been of no service in these cases, on 

 account of the diameter of the optic pencil, which, in the 20 feet 

 telescope, at the time of sweeping, was no more than ,12 inch. 

 The effect of this increased sensibility was such, that if a star of 

 the 3d 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, have generally been declined 

 in my sweeps, even with the 20 feet telescope. And I remem- 

 ber, 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, increas- 

 ing 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. Such 

 striking effects are a sufficient proof of the great sensibility of 

 the eye, acquired by keeping it from the light. 



On taking notice, in the beginning of sweeps, of the time that 

 passed, I found that the eye, coming from the light, required 



