VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 583 



inequality will take place. The equation of light, in this present sense therefore, 

 is cn = /. Now, since we know that the density of light decreases in the ratio 

 of the squares of the distances of the luminous objects, the expression for its quan- 

 tity at the distance of the observer d, will be — . 



In natural vision, the quantity / undergoes a considerable change, by the opening 

 and contracting of the pupil of the eye. If we call the aperture of the iris a, we 

 find that in different persons it differs considerably. Its changes are not easily to 

 be ascertained; but we shall not be much out in stating its variations to be chiefly 

 between J and 2 tenths of an inch. Perhaps this may be supposed under-rated; 

 for the powers of vision, in a room completely darkened, will exert themselves in 

 a very extraordinary manner. In some experiments on light, made at Bath, in the 

 year 1780, I have often remarked, that after staying some time in a room fitted up 

 for these experiments, where on entering I could not perceive any one object, I 

 was no longer at a loss, in half an hour's time, to find every thing I wanted. It 

 is however probable that the opening of the iris is not the only cause of seeing 

 better after remaining long in the dark ; but that the tranquillity of the retina, 

 which is not disturbed by foreign objects of vision, may render it fit to receive im- 

 pressions 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 heavens, of 4, 5, or 6 hours duration, that the sensibility of 

 the eye, in consequence of the exclusion of light from surrounding objects, by 

 means of a black hood which I wear on these occasions, has been very great; and 

 it is evident, that the opening of the iris would have been of no service in these 

 cases, on account of the diameter of the optic pencil, which in the 20 feet teles- 

 cope, at the time of sweeping, was no more than .12 inch. The effect of this in- 

 creased 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 

 remember, that after a considerable sweep with the 40 feet instrument, the appear- 

 ance of Sirius announced itself, at a great distance, like the dawn of the morning, 

 and came on by degrees, increasing in brightness, till this brilliant star at last en- 

 tered 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 near 20 m , before it could be 

 sufficiently reposed to admit a view of very delicate objects in the telescope; and that 

 the observation of a transit of a star of the 2d or 3d magnitude would disorder the eve 

 again, so as to require nearly the same time for the re-establishment of its tranquillity. 



