penetrating into Space by Telescopes. 67 



observations themselves, I shall select a series of them for that 

 purpose, and relate them in the order which will be most illus- 

 trating. 



First, with regard to the eye, it is certain that its power, like 

 all our other faculties, is limited by nature, and is regulated by 

 the permanent brightness of objects ; as has been shewn already, 

 when its extent with reflected light was compared to its exer- 

 tion on self-luminous objects. It is further limited on borrowed 

 light, by the occasional state of illumination ; for, when that 

 becomes defective at any time, the power of the eye will then 

 be contracted into a narrower compass ; an instance of which is 

 the following. 



In the year 1776*, when I had erected a telescope of 20 feet 

 focal length, of the Newtonian construction, one of its effects 

 by trial was, that when towards evening, on account of dark- 

 ness, the natural eye could not penetrate far into space, the 

 telescope possessed that power sufficiently to shew, by the dial 

 of a distant church steeple, what o'clock it was, notwithstanding 

 the naked eye could no longer see the steeple itself. Here I only 

 speak of the penetrating power ; for, though it might require 

 magnifying power to see the figures on the dial, it could 

 require none to see the steeple. Now the aperture of the tele- 

 scope being 12 inches, and the construction of the Newtonian 

 form, its penetrating power, when calculated according to the 



x/Z 



given formula, will be - —^221112 JZiL. — 38,99. A, b, and a, 

 being all expressed in tenths of an inch.* 



* I have given the figures, in all the following equations of the calculated pene- 

 trating powers, in order to shew the constructions of my instruments to those who may 

 wish to be acquainted with them. 



K2 



