Fig. 154.—Image on the Retina.
We have in the previous chapter mentioned the effect of light upon the eye and its direction, and when an object is placed very near the eye we know it cannot be distinctly seen; a magnified image is thrown upon the retina, and the divergency of the rays prevents a clear image being perceived. But if a small lens of a short “focal length” be placed in front of the eye, having PQ for its focus, the rays of light will be parallel, or very nearly so, and will as such produce “distinct vision,” and the image will be magnified at pq. In the Compound Refracting Microscope, BAB is the convex lens, near which an object, PQ, is placed a little beyond its focal length. An inverted image, pq, will then be formed. This image is produced in the convex lens, bab′, and when the rays are reflected out they are parallel, and are distinctly seen. So the eye of the observer at the point E will see a magnified image of the object at PQ brought up to pq (fig. 155).
Fig. 155.—The Microscope lenses.
Sir Isaac Newton suggested the Reflecting Microscope, and Dr. Wollaston and Sir David Brewster improved the instrument called the “Periscopic Microscope,” in which two hemispherical lenses were cemented together by the plane surfaces, and having a “stop” between them to limit the aperture. Then the “Achromatic” instrument came into use, and since then the Microscope has gradually attained perfection.
Fig. 156.—Concave lens.
We have so frequently mentioned lenses that it may be as well to say something about them. Lenses may be spherical, double-convex, plane-convex, plane-concave, double-concave, and concave-convex. Convex lenses bring the parallel lines which strike them to a focus, as we see in the “burning-glass.” The concave or hollow lens appears as in fig. 156. The rays that follow it parallel to its axis are refracted, and as if they came from a point F in the diagram. But converging rays falling on it emerge in a parallel direction as above, or diverge as in fig. 158.
Fig. 157.
1. Focus of parallel rays. 2. Focus of divergent rays. 3. Focus of divergent rays brought forward by more convex lens.