penetrating into Space by Telescopes. 53 



every part of CN which is not applied for the purpose of vision, 

 L representing the whole quantity of light thrown out by CN, 

 we shall denote that part of it which is used in vision, either by 

 the eye or by the telescope, /. This will render the conclusions 

 that may be drawn hereafter more unexceptionable ; for, the 

 quantity of light / being scattered over a small space in propor- 

 tion to L, it may reasonably be looked upon as more uniform 

 in its texture ;• and no scruples about its inequality will take 

 place. The equation of light, in this present sense, therefore, is 

 CN—L 



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 quantity at the distance of the observer 



D, will be -jjr. 



In natural vision, the quantity I 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 varia- 

 tions to be chiefly between 1 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, 1 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 



