56 



Microscopical Essays. 



three inches long, on the board D ; draw three lines with ink on 

 this piece of paper, fo as to divide it into four equal parts, taking 

 care that the middle of the paper correfponds with the center of 

 the lens. There is alfo a Hiding eye-piece, which is reprefented 

 at e. 



Take this apparatus into the darker! part of the room, but op- 

 pofite to the window ; direct the glafs towards any remarkable 

 and diftant object which is out of doors, and move the Aiding 

 piece B, until the image of the object on the paper is fharp and 

 clear. The diftance between the face of the paper and the lens, 

 (which is fhewn on the fide of the rule by the divifions thereon) 

 is the focus of the glafs ; now fet the eye-piece e E to the fame 

 diflance on the other fide of the glafs, then with one eye clofe to 

 the light at e, look at the magnified image of the lines, and 

 with the other eye at the lines themfelves : the image, feen by 

 means of the glafs, and expreffed in the figure by the dotted 

 lines, will be double the breadth of the fame object feen by the 

 natural eye. This will be found to be true, whatfoever is the focus 

 of the lens with which the experiment is made. 



This experiment is rendered more fimple to thofe who are not 

 accuftomed to obferve with both eyes at the fame time, by 

 making ufe of half a lens, and placing the diameter perpendicular 

 to the rule, as they may then readily view the magnified image 

 and real object with the fame glance of the eye, and thus com- 

 pare them together with eafe and accuracy. 



Let the angle A F B, Fig. 3, Plate II. A, reprefent that which is 

 formed at the naked eye, by the rays of light which pafs from the 



extremities 



