optics, 239 



and </K,eE,/L, from the point A; and there- 

 fore there will be no picture formed behind the 

 glass. 



If the focal distance of the glass, and the dis- 

 tance of the object from the glass, be known, the 

 distance of the picture from the glass may be 

 found by this rule ; viz. multiply the distance of 

 the focus by the distance of the object, and divide 

 the product by their difference ; the quotient will 

 be the distance of the picture. 



The picture will be as much larger, or less than 

 the object, as its distance from the glass is greater 

 or less than the distance of the object ; for as B e 

 is to e by so is A C to ca; so that if A B C be the 

 object, c b a will be the picture ; or if c b a be the 

 object, ABC will be the picture. 



If rays converge before they enter a convex lens, 

 they are collected at a point nearer to the lens than 

 the focus of parallel rays. If they diverge before 

 they enter the lens, they are then collected in a 

 point beyond the focus of parallel rays ; unless 

 they proceed from a point on the other side at the 

 same distance with the focus of parallel rays ; in 

 which case they are rendered parallel. 



If they proceed from a point nearer than that, 

 they diverge afterwards, but in a less degree than 

 before they entered the lens. 



When parallel rays, as a be de (Plate 13. fig. I.), 

 pass through a concave lens, as A B, they will di- 

 verge after passing through the glass, as if they 

 had come from a radiant point C, in the centre of 

 the convexity of the glass ; which point is called 

 virtual or imaginary focus. 



Thus, the ray a, after passing through the glass 

 A B, will go on in the direction k /, as if it had 

 proceeded from the point C, and no glass been in 





