Chap, xxv.] FORMATION OF IMAGES BY LENSES. 317 



xx fall on the lens ; they will be refracted to meet at 

 F', at a distance from the lens equal to the focal dis- 

 tance. At that point the rays will cross, and if con- 

 tinued, diverge. From A draw a secondary axis AC. 

 If prolonged it will 

 meet the refracted 

 ray at a. Thus, a 

 pencil or cone of rays 

 passing from the ^' 



point A will have its Fig. 145. Formation of a Eeal linage by a 



conjugate focus at a, Couvex Lens - 



and thus a will be the 



image of A. Similarly, draw a secondary axis from B ; it 

 will intersect the refracted ray from B at 6. A cone of 

 rays from B will find its conjugate focus at b, and b will 

 be the image of B. Each point of AB will have pro- 

 ceeding from it as a focus a pencil of rays, which will 

 find its conjugate focus between a and b. Thus an 

 image ab of the arrow AB is formed. It is a real 

 image, that is, on the opposite side from the object, 

 and is inverted and smaller than the object. Should ab 

 be the object, then AB would be the image. This 

 follows from the relation between conjugate foci. 

 From what has been seen about conjugate foci, it also 

 follows that the nearer AB approaches to the focal 

 distance from the lens, the farther ab recedes from 

 the lens and the larger it becomes ; while the farther 

 AB is from the lens the nearer is the image to the 

 focal distance, and the smaller it is. To put it in 

 another way, the image of an object placed at a much 

 greater distance from the lens than the focal length is a 

 real image, very small and inverted, and in the neigh- 

 bourhood of the focal distance, while the image of an 

 object placed very near to the focal distance of the lens, 

 yet outside of it, is still a real image and inverted, but 

 much larger than the object, and far beyond the focal 

 distance. 



