318 Sir William Herschei/s observations and experiments 
ter south of vj Herculis, and the cluster north preceding e 
Pegasi. But their distances cannot be ascertained by the 
method of equalising starlight : their probable situation in 
space may however be deduced from telescopic observations. 
To these very faintly visible objects may be added two of 
a different nature, namely, the nebulosity in the sword of 
Orion, and that in the girdle of Andromeda. 
VII. Of the extent of telescopic vision. 
The powers of telescopes to penetrate into the Profundity 
of space is the result of the quantity of light they collect and 
send to the eye in a state fit for vision. The method of cal- 
culating the quantity of this power has been fully explained 
in a Paper read before the Royal Society, November 21, 
1799 ; and the formulae which have been given in that Paper 
have already been applied to show to what extent this power 
has been carried in the telescopes I used for astronomical 
observations. The calculated results, however, give this 
power only in reference to that of natural vision, and the 
uncertainty in which we were left with regard to its extent, 
was equally thrown over that of telescopic vision. 
The equalisation of starlight, when carried to a proper de- 
gree of accuracy, will do away the cause of the error to which 
the telescopic extent of vision has been unavoidably subject, 
we may therefore safely apply this vision to measure the 
Profundity of sidereal objects that are far beyond the reach 
of the natural eye; but for this purpose the powers of pene- 
trating into space of the telescopes that are to be used must 
be reduced to what may be called gaging powers; and as 
