MAGNIFYING GLASSES. 
mere inspection of the figure, that the effect of the lens upon 
the rays will he precisely the same, wherever the point o may 
be placed; this lens, therefore, gives a large field equally 
well defined in all directions, and 
since it is no matter in what 
position it is held, it is very con¬ 
venient as a hand and pocket glass; 
it is usually mounted in a small 
case, such as is shown in fig. 11, 
which can he carried in the waist¬ 
coat pocket. 
27. Magnifying glasses of low powers, such, for example, as 
those which range from 5 to 40, may be constructed with much 
advantage in one or the other of the above forms. When, how¬ 
ever, higher powers are necessary, the use of such lenses, with very 
short focal length, is attended with much practical inconvenience, 
which has been removed by the use of magnifiers, consisting of 
two or more lenses combined. The combinations of this kind which 
are found most efficient, consist of two or three plano-convex 
lenses, with their convex side towards the eye; these are called 
doublets and triplets. 
28. After what has been explained in our Tract upon Optical 
Images, the principle upon which these magnifiers depend will 
be easily understood. 
Let e e and d d, fig. 12, represent the two lenses of a doublet, 
and let o o be a small object placed before D n, at a distance from 
it less than its focal length. According to what has been ex¬ 
plained, d d will produce an imaginary image of o o at i i, more 
distant from 3) n than o o , so that an eye placed behind n d would 
receive the rays from o o, as if they had diverged from the corre¬ 
sponding points of i i. 
But instead of being received by an eye placed behind d d, 
these rays are received by the other lens E e ; the image i i there¬ 
fore plays the part of an object before the lens e e, and being at a 
distance from E E less than the focal length of the latter, an 
imaginary image of i i will be produced at 1 1 ; the rays, after 
passing through e E, entering the eye as if they had come from 
the corresponding points of 11. 
To cut off all scattered rays not necessary for the formation of 
the image, a stop or diaphragm, s s, consisting of a circular disc of 
metal, with a hole in its centre, is interposed between the two 
lenses. 
29. Such a combination, when high powers are necessary, has 
several advantages over an equivalent single lens. In the first 
place, the effect of spherical aberration is much less, and secondly, 
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