3i 8 PHYSIOLOGICAL PHYSICS. tchap. xxv. 



Now consider what occurs when the object is nearer 

 to the lens than the focal distance. Let AB (Fig. 1 46) be 

 such an object. The cone of rays from A, namely, a', no 

 longer converge after passing through the lens, but 



are still divergent. They 

 have, therefore, no conju- 

 gate focus on the opposite 

 side of the lens from A. 

 If prolonged backwards, 

 however, they will meet in 

 a, which is, therefore, the 



Fig. 146. Formation of a Vir- Conjugate foCUS of A, and 



tuaJ image by a Convex Lens. on the same side, a virtual 



focus, therefore. Similarly, 



the pencil of rays from B after refraction is still a 

 divergent pencil b', and has no real conjugate focus, 

 but a virtual one in b. Each point between A and B 

 has also its virtual conjugate focus, and thus there is 

 formed a virtual image of AB, namely, ab, and this 

 virtual image is erect and larger than the object. The 

 nearer the object is to the focal distance, if still inside 

 of it, the larger will be the virtual image produced. 



CONCAVE LENSES have only virtual images, which 

 are erect and smaller than the object. This is evident 

 from the fact that the conjugate focus of a concave 

 lens is virtual. 



Size of image formed by convex lens. 

 The proportion between the sizes of image and object 

 is directly as the proportion between the distances of 

 the t\\ o from the lens. Thus, 



size of object distance of object from lens 

 size of image distance of image from lens 



If AB be the image, ab the object, p the distance of 

 the former from the lens, and p the latter distance, 



AB p 



' ' 



p 

 AB = ab X ~/ 



