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IV. On the Power of penetrating into Space by Telescopes ; with 

 a comparative Determination of the Extent of that Power in 

 natural Vision, and in Telescopes of various Sizes and Con- 

 structions; illustrated by select Observations. By William 

 Herschel, LL.D. F.R.S. 



Read November 21, 1799. 



It will not be difficult to shew that the power of penetrating 

 into space by telescopes is very different from magnifying 

 power, and that, in the construction of instruments, these two 

 powers ought to be considered separately. 



In order to conduct our present inquiry properly, it will be 

 necessary to examine the nature of luminous bodies, and to 

 enter into the method of vision at a distance. Therefore, to 

 prevent the inaccuracy that would unavoidably arise from the 

 use of terms in their common acceptation, I shall have recourse 

 to algebraic symbols, and to such definitions as may be necessary 

 to fix a precise meaning to some expressions which are often 

 used in conversation, without much regard to accuracy. 



By luminous bodies I mean, in the following pages, to denote 

 such as throw out light, whatever may be the cause of it : even 

 those that are opaque, when they are in a situation to reflect 

 light, should be understood to be included ; as objects of vision 

 they must throw out light, and become intitled to be called 

 luminous. However, those that shine by their own light may 



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