. The microscope and its revelations. FIG. 15.—Rays diverge when a radiant is placed between a lens and itsprincipal focus. Focus of divergent rays is virtual. meet in a point. This is called the ri/ttnil conjugate focus of theradiant. The principal focus of a concave (or diverging) lens isshown in fig. 16. It will be seen that the principal focus is notreal but Parallel rays falling on a concave lens are rendered. FIG. 16.— Virtual focus of concave lens. divergent on the other side of the lens, and consequently can nevercome to a focus. But if we trace these divergent rays backwards
. The microscope and its revelations. FIG. 15.—Rays diverge when a radiant is placed between a lens and itsprincipal focus. Focus of divergent rays is virtual. meet in a point. This is called the ri/ttnil conjugate focus of theradiant. The principal focus of a concave (or diverging) lens isshown in fig. 16. It will be seen that the principal focus is notreal but Parallel rays falling on a concave lens are rendered. FIG. 16.— Virtual focus of concave lens. divergent on the other side of the lens, and consequently can nevercome to a focus. But if we trace these divergent rays backwards,as in the dotted lines of fig. 16, we find that they meet in a point,and this point is called the rirtini/ />rinci/>til focus of the lens. It will he manifest that since the rays in passing through lenses ofvarious kinds arc unequally refracted they cannot all meet exactly in asingle local point. This gives rise to what is a most important featurein the behaviour of lenses, which is known as spherical aberration. Figs. 17 and IK show the refraction of rays of monochromatic1 A fro/ iniii^c l>c received on a screen, but ft virtual image cannot. SPHERICAL ABERRATION 15 light parallel to the axis falling on a plano-convex lens of crownglass. These figures illustrate : (1) Longitudinal spherical aberrationand (2) the focal length of a plano-convex lens and the point fromwhich it is measured. (1) In regar
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
