LEASE'S. 
28. Examples of each of these forms of lenses are more or less 
familiar to every one. Thus the glasses of spectacles- used by 
Fig. 18. 
weak-sighted or aged persons, are usually double convex lenses. 
Those used by short-sighted persons are generally double concave 
lenses. 
Spectacles called periscopic are sometimes used. The glasses 
of these, which 
are suited to weak 
sight, are me¬ 
niscus, and those 
adapted to short 
sight are concavo- 
convex lenses. 
The eye-glasses 
of opera-glasses 
are usually double concave lenses.. The object glasses are 
generally plano-convex lenses, the plane side being turned 
inwards. 
29. If an object such as o"', fig. 21, be placed before a convex 
lens, and at right angles to its axis, an image, i"', of it will be 
produced behind 
the lens, also at 
right angles to the 
axis, inverted in 
position in relation 
to the object, that 
is, the top of the 
image corresponding 
with the bottom of 
the object, and the right side with the left, and vice versa. 
If the object be placed near the lens, the image will be formed 
at a great distance from it, and will be greater than the object in 
its linear dimensions in the same proportion as its distance is 
greater than that of the object from the lens. 
This will be evident by inspecting the figure. The length of 
the image, T", is evidently greater than that of the object, o'", in 
Fig. 20. 
Fig. 19. 
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