gio Sir William Herschel’s observations and experiments 
Profundity of space to which our telescopes might reach, gave 
rise to an investigation of their space penetrating power ; and 
finding that this might be calculated with reference to the 
extent of the same power of which the unassisted eye is 
capable, there always remained a desideratum of some sure 
method by which this might be ascertained. 
Of various experiments I have long ago tried, the equali- 
sation of starlight, which about four years ago I began to put 
into execution, appeared to be the most practicable. A de- 
scription of the apparatus and the method of making use of 
it is as follows. 
Of ten highly finished mirrors I selected two of an equal 
diameter and focal length, and placed them in two similarly 
fitted up seven feet telescopes. When they were completely 
adjusted, I directed them both, with a magnifying power of 
118, to the same star, for instance, Arcturus : and upon trial 
I found the light not only of this, but of every other star to 
which they were directed, perfectly equal in both telescopes. 
The two instruments, when I viewed the stars, were placed 
one a little before the other, and so near together that it 
\yould require little more than one second of time to look 
from one into the other. This convenient situation of the 
instruments is of great importance. The impression of the 
light made by the view of one star should be succeeded as 
soon as possible by the view of the other ; and these alter- 
nate inspections should also be many times repeated, in order 
to take away some little advantage which the last view of a 
bright object has over that immediately preceding. 
In comparing the light of one star with that of another, I 
laid it down as a principle, that no estimation but that of per- 
