There are two classes of Telescopes—the reflecting and refracting. Lord Rosse’s is an instance of the former. Mr. Grubb’s immense instrument in Dublin is a refractor. The Microscope has been also attributed to Zacharias Jansen, and Drebbel, in 1619, possessed the instrument in London, but it was of little or no use. The lens invented by Hall, as already mentioned, gave an impetus to the Microscope. In the simple Microscope the objects are seen directly through the lens or lenses acting as one. The compound instrument is composed of two lenses (or a number formed to do duty as two), an eye-lens, and an object-lens. Between these is a “stop” to restrain all light, except what is necessary to view the object distinctly. The large glass near the object bends the rays on to the eyeglass, and a perfect magnified image is perceived. We annex diagrams, from which the construction will be readily understood. [Illustration: 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). [Illustration: 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. [Illustration: 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.