Microscopical Essays, 



be fo refracted by the lens, that after going out of it, they will 

 become parallel. If the candle be placed nearer the lens than it's 

 focal diftance, the rays will diverge more or lefs, as the candle is 

 more or lefs diftant from the focus. 



If any objed, A B, Fig. 7, Plate I. be placed beyond the focus 

 of the convex lens E F, fome of the rays which flow from every 

 point of the objecl, on the fide next the glafs, will fall upon it, 

 and after patting through it, they will be converged into as many 

 points on the oppofite fide of the glafs ; for the rays a b, which 

 flow from the point A, will converge into ab, and meet at C. 

 The rays c d, flowing from the point G, will be converged into 

 c d, and meet at g ; and the rays which flow from B, will meet 

 each other again at D ; and fo of the rays which flow from any of 

 the intermediate points : for there wilt be as many focal points 

 formed, as there are radiant points in the object, and confe- 

 quently they will paint on a ftieet of paper, or any other light- 

 coloured body, placed at D g c, an inverted image of the objecl. 

 If the objecl: is brought nearer the lens, the piclure will be formed 

 further off. If it be placed at the principal focus, the rays will 

 go out parallel, and confequently form no piclure behind the 

 glafs. 



The human eye is fo eonflituted, that it can only have diftlncl 

 virion, when the rays which fall on it are parallel, or nearly fo; 

 becaufe the retina, on which the image is painted, is placed in 

 the focus of the cryftalline humor, which performs the office of 

 a lens in collecting rays, and forming the image in the bottom 

 of the eye. 



The 



