the expelled Return of a Comet . 43 \ 
though ftill fufficiently vifible. Let us fuppofe this to have 
been a telefcope of 9-feet focal length, with an aperture of 1,65 
inch; then, becaufe the diameter of the aperture of a tele- 
fcope fufficient to render the comet equally vifible ffiould be as 
the product of its diftances from the fun and earth, and the 
product of the numbers above-mentioned 0,986 and 1,37 is 1,35^ 
we (hall have the following analogy to find the aperture of a 
jrefra&ing telefcope fufficient to fhew the comet as it appeared 
to Hevelius. As 1,35 : 1,65 inch 9 : 11 inches, fo is the 
product of diftances from the fun and earth to* the diameter of 
the aperture required in inches. 
