CJ4 'Method to determine the 



it will give 140 for the magnifying power. Again, fuppofe 

 with fome other eye-piece the beam of light fills the lines at 

 the fourth divifion : divide your 2 1 o by 4, and it will give 5 a '5 

 or 52, for the power. If your paper be oiled with good 

 linfeed oil that has been boiled, fuch as the painters call 

 drying oil, it will laft for many years, and have this advan- 

 tage over the inftruments made for this purpofe which are 

 fold for3l. 3s. that, as the fcrew for receiving the dark glafs 

 projefts beyond the place where the paper lliould be held, 

 the paper will bend, and thus can be introduced into its 

 proper fituation without producing any fenfible error. An- 

 other method, which I fometimes ufe in confirmation of the 

 former, and which cannot fail to produce full conviftlon on 

 the mind, is as follows : 'I meafure the fpace occupied by a 

 number of the courfes of brick in fome modern brick build- 

 ing, which, upon an average, will be found to have eight 

 courfes in two feet : hence each courfe is three inches. I 

 then cut a piece of paper three inches in height, and nearly 

 of the length of a brick, which is about nine inches, fo that 

 it may reprefent a brick, but with this advantage, that its 

 edges will be much better defined ; and fixing the paper 

 againft the wall (fee fig. 27), place the telefcope to be ex- 

 amined at the diftance of 50 or 100 yards from the wall; 

 and looking through the telefcope at the paper with 

 one eye, and at the fame time with the other eye look- 

 ing paft the telefcope, I obferve what extent of wall the 

 magnified image of the paper appears to cover. I then 

 count the courfes of bricks in that extent, which gives the 

 magnifying power of the telefcope. This method will ap- 

 pear a little difficult at firft, but, by a few trials, will be- 

 come fo familiar, that the paper, as feen by the telefcope, 

 may be eafily compared with the bricks fccn by the other 

 eye. 



I Intended to have added feveral other obfervations on 

 fubje£ts nearly related to the above : but having exceeded 



the 



J 



