38 Microscopical Essays. 



the glafs in the direction D E ; but when it comes to E, it will 

 be as much refracted the contrary way. If the ray of light had 

 fallen perpendicularly on the fur face of the plane glafs, it would 

 have paffed through it in a flrait line, and not have been 

 refra&ed at all. 



If parallel rays of light, as a b c d e f g, Fig. 6, Plate I. fall 

 directly upon a convex lens A B, they will be fo bent, as to 

 unite in a point C behind it. For the ray d D, which falls per- 

 pendicularly upon the middle of the glafs, will go through it 

 without fuffering any refraction : but thofe which go through the 

 fides of the lens, falling obliquely on it's furface, will be fo bent, 

 as to meet the central ray at C. The further the ray a is. from 

 the axis of the lens, the more obliquely it will fall upon it. The 

 rays a b c d e £gswill be fo refra&ed, as to meet or be collect ed in 

 the point K, called the principal focus, whofe diftance, in a 

 double convex lens, is equal to the radius or femi-diameter of the 

 fphere of the convexity of the lens. All the rays crofs the middle 

 ray at C, and then diverge from it to the contrary fide, in the 

 fame manner as they were before converged. 



If another lens, of the fame convexity, as A B, Fig. 6, Plate I. 

 be placed in the rays, and at the fame diflance from the focus, 

 it mil refract, them, fo that after going out of it, they will all 

 be parallel again, and go on in the fame manner as they came to 

 the ftrft glafs A B, but on the contrary fides of the middle ray. 



The rays diverge from any radiant point, as from a principal 

 focus : therefore, if a candle be placed at C, in the focus of the 

 convex lens AB, Fig. 6, Plate I. the rays diverging from it will 

 6 be 



