MAGNIFYING GLASSES. 



mere inspection of the figure, that the effect of the lens upon 

 the rays will be precisely the same, wherever the point o may 

 bo 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 be 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 D, 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 D D than o o, so that an eye placed behind D D would 

 receive the rays from o o, as if they had diverged from the corre- 

 sponding points of * 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 I I ; the rays, after 

 passing through E E, entering the eye as if they had come from 

 the corresponding points of 1 1. 



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, 



108 



