CHAPTER I 



USE OF THE HAND MAGNIFIER 



Corrected Lenses. — Corrected hand magnifiers (Figs. 

 1 and 2) are made normally of either two or three simple 

 component lenses cemented together by a transparent 

 resin. They are composed of two different kinds of glass, 

 and the components are of opposite shapes, being con- 

 vergent or divergent in their action on plane light waves. 

 Such a cemented combination will be called here, following 



Fig. 1. Fig. 2. 



Fig. 1. Diagrammatic section of eye with low-power corrected magnifying 

 lens. This figure shows how the aperture of the pencil or cone of light from any 

 point of the object (in this ease, the center) is bounded by the iris of the eye. 

 The flat side of the lens is next the eye. (Flint glass is dotted, crown glass 

 cross-lined.) 



Fig. 2.' — Diagrammatic section of the eye with a low-power triplet magnifier. 

 Cone of light from center point of object. 



good authority, a doublet or triplet. (The old use of the 

 term "doublet" or "triplet" for two or three uncorrected 

 lenses placed near together will not be followed here, since 

 such arrangements have ceased to be employed for most 

 scientific purposes.) Such a combination may be calcu- 

 lated (by choosing the refractive indices of the glasses and 

 varying the radii of curvature) to correct the lens more or 

 less perfectly, not only for the chromatic errors inseparable 

 from all refraction of white light; but also, which is at 



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