The microscope; an introduction to microscopic methods and to histology . of all rays traversing the centerof the lens. Thick Lenses.—In all of the diagrams of lenses and the course of rays throughthem in this book the lenses are treated as if they were infinitely thin. In thicklenses like those figured, while there would be no angular deviation for rays trav-ersing the center of the lens, there would be lateral displacement. This is shownin Fig. 57 illustrating the effect of the cover-glass. \ 5. Secondary Axis.—Every ray traversing the center of the lens, except theprincipal axis, is a secon
The microscope; an introduction to microscopic methods and to histology . of all rays traversing the centerof the lens. Thick Lenses.—In all of the diagrams of lenses and the course of rays throughthem in this book the lenses are treated as if they were infinitely thin. In thicklenses like those figured, while there would be no angular deviation for rays trav-ersing the center of the lens, there would be lateral displacement. This is shownin Fig. 57 illustrating the effect of the cover-glass. \ 5. Secondary Axis.—Every ray traversing the center of the lens, except theprincipal axis, is a secondary axis ; and every secondary axis is more or lessoblique to the principal axis. In Fig. 14, line (2), is a secondary axis, and in , line (1). See also Fig. 58. Figs. 10, 11.—Sectional views of aconcave or diverging and a convex orconverging lens to show that in the con-cave lens the principal focus is virtual asindicated by the dotted lines, while withthe convex lens the focus is real and onthe side of the lens opposite to that fromwhich the light \ 6. Principal Focus.—This is the point where rays parallel with the axis andtraversing the lens cross the axis ; and the distance from the focus to the center ofthe lens measured along the axis is the Principal Focal Distance. In the diagrams,Fig. 10 is seen to be a diverging lens and the rays cross the axis only by being pro-jected backward. Such a focus is said to be virtual, as it has no real existence. In MICROSCOPE AND ACCESSORIES \CH. I Fig. 11 the rays do cross the axis and the focus is said to be real. If the lightcame from the opposite direction it would be seen that there is a principal focuson the other side, that is there are two principal foci, one on each side of the two foci are both principal foci, but they will be equally distant from thecenter of the lens only when the curvature of the two lens surfaces are may be foci on secondary axes also, each focus on a secondary
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