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 
Fig. 24. 
lens, whose focal length is b f, and let l m he 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 M, 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 l 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 l 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 
l m, without the interposition of a lens, and the eye will be affected, 
and vision produced exactly as if such an object as l 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 
heal 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 
IMAGINAEY. 
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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