. The microscope and microscopical methods. Microscopes; 1896. Fig. 12.—Double Convex Lens, Showing Chromatic Aberration. The ray of white light (a/) is represented as dividing into the short waved, blue (b) and the long waved, red (r) light. The blue (b) ray comes to a focus nearer the lens and the red ray {r) farther from the lens than the principal focus (/). Principal focus {f) for rays very near the axis, f andf", foci of blue and red light coming from near the edge of the lens. The intermediate wave lengths would have foci all the way between f andf". \ 7. Chromatic Aberration.
. The microscope and microscopical methods. Microscopes; 1896. Fig. 12.—Double Convex Lens, Showing Chromatic Aberration. The ray of white light (a/) is represented as dividing into the short waved, blue (b) and the long waved, red (r) light. The blue (b) ray comes to a focus nearer the lens and the red ray {r) farther from the lens than the principal focus (/). Principal focus {f) for rays very near the axis, f andf", foci of blue and red light coming from near the edge of the lens. The intermediate wave lengths would have foci all the way between f andf". \ 7. Chromatic Aberration.—This is due to the fact that ordinary light consists of waves of varying length, and as the effect of a lens is to change the direction of the waves, it changes the direction of the short waves more markedly than the long waves. Therefore the short waved, blue light will cross the axis sooner than the long waved, red light, and there will result a superposition of colored images, none of which are perfectly distinct. (Fig. 12). Fig. 13. The ray (o) near the edge of the lens is brought to a focus nearer the lens than ike ray(i). Both are brought to a focus sooner than rays very near the axis, if) Princi- pal focus for rays very near the axis ; (f) Focus for the ray (i), and {f") Focus for the ray (o). Intermediate rays would cross the axis all the way from [f" tof).. Fig. 13. Double Convex Lens, showing Spherical Aberration. § 8. Spherical Aberration.—This is due to the unequal turning of the light in different zones of a lens. The edge of the lens refracts proportionally too much and hence the light will cross the axis or come to a focus nearer the lens than a ray which is nearer the middle of the lens. Thus, in Fig. 13, if the focus of parallel rays very near the axis is at/, rays {0 z) nearer the edge would come to a focus nearer the lens, the focus of the ray nearest the edge being nearest the lens. Every simple lens has the defect of both chromatic and sph
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