Anatomy and physiology of the eye, with hints for the preservation of the eyesight (1901)
Herbert, John Frederick, 1860- · Philadelphia, Pa., The author · leaf 56
Fig;nre 30.Parallel rays are brniii^ht to a focus at F, on opposite side of lens. This point is therefore called the principal focus of thelens L. The space intervening between the lens and itsprincipal focus, F, will depend upon the degree of curva- CONCAVE LENS. … Figure 31.Parallel rays are brought to a virtual focus at F, 011 same side of lens. ture of the lens. If the distance between ly and F is onemetre, the lens has a focus of one metre, or as commonh^called, it has the strength of one diopter. Such a lens isemployed as a unit for calculation. When…
Eye · Lenses
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