OPTICAL IMAGES. 



similar object at a greater distance in front of the lens, and of 

 proportionally greater dimensions. 



To render this more clear, let A c, fig. 24, represent a convex 



lens, whose focal length is B F, and let L M be an object placed 

 before it at a less distance than B F. Now, it will be understood 

 that from every point of the object L u, rays of light diverge, 

 which, passing through the lens A B, have their directions changed 

 by it, and this change is such that, instead of diverging from 

 the various points of the object L M, they will diverge from a 

 similar series of points placed at a greater distance before the lens. 

 In fine, after passing through the lens, they will diverge as if they 

 had issued from the points of an object I m in all respects similar 

 to the object L M itself, and having a like position, but greater 

 than the object in its linear dimensions, in the proportion of I B to 

 L B ; that is, of its distance from the lens to the distance of the 

 object from the lens. 



In this case, then, no actual optical image is produced which, 

 as in the former case, can be received and exhibited upon a card. 

 But if the eye of an observer be placed behind the lens, it will 

 receive the rays proceeding from the object L M, and passing through 

 the lens exactly as if they really had proceeded from the object 

 I m, without the interposition of a lens, and the eye will be affected, 

 and vision produced exactly as if such an object as / m were present. 



35. When the optical image is actually formed, so that it can 

 be received and exhibited upon a card or screen, it is said to be a 

 BEAL IMAGE ; and when it is formed in the manner above described, 

 so as to be seen by the eye directly receiving the rays from the 

 lens, but not capable of being formed on a screen, it is said to be 



IMAGINARY. 



An exception might be taken to the terms, inasmuch as the 

 visual image is as real in the one case as in the other. They have, 

 however, been generally adopted in the nomenclature of optics. 



All that has been said of the optical images, real and ima- 

 ginary, produced by double-convex lenses, and of their principal 

 foci, will be equally applicable to plano-convex and meniscus 

 lenses. In each of these the convexity being the prevalent 

 character, their optical effects are similar to those of double- 

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