. A treatise on the diseases of the eye. an infinitedistance, the smallest inverted image will be formed behind the lens at itsprincii)al focus. (2) If the object be approximated so as to lie at doublethe focal length of the lens, its inverted image will be situated at double thefocal length behind the lens, and be the same size as the object. (3) Ifthe object be brought still closer, but yet further than the anterior focus,the inverted image will move further away from the lens and be larger thanthe object. (4) If the latter be placed at the anterior focus, no real imagewill be formed, for th
. A treatise on the diseases of the eye. an infinitedistance, the smallest inverted image will be formed behind the lens at itsprincii)al focus. (2) If the object be approximated so as to lie at doublethe focal length of the lens, its inverted image will be situated at double thefocal length behind the lens, and be the same size as the object. (3) Ifthe object be brought still closer, but yet further than the anterior focus,the inverted image will move further away from the lens and be larger thanthe object. (4) If the latter be placed at the anterior focus, no real imagewill be formed, for the rays will issue from the lens in a parallel direction.(5) If the object is placed inside the focal length, the rays will still issue ina divergent direction from the lens, and the latter will act as a magnifyingglass, the image will not be inverted and situated behind the lens, but willbe erect, magnified, and situated in front of the lens, t. f., on the same sideas the object. Fig. 192 will explain this. If ^4 B be an object situated. closer to the lens I than its anterior focus F, the rays from A will still divergeafter their passage through the lens, and in such a direction as if they camefrom a, and the rays from B will diverge as if they came from b. If theeye E is placed (m the other side of the lens, it will see, instead of the objectA B, its magnified, erect image, a h. This magnifying power of the lens will be greater according to the short-ness of its focal length, thus a four-inch lens magnifies more than a five-inch,and the latter more than a six-inch lens. In order therefore to give the cor-rect magnifying power, and to demonstrate at once that a six-inch lens mag-nifies less than a five-inch, we designate the magnifying power of a lens byfractions, the numerators of which are one, the denominators, the focallength of the lens. Thus one-fourth is stronger than one-fifth, the latterfraction being less than the former. Moreover, this way of expressing thestrength o
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