A somewhat more complicated form of lens is called the “Meniscus,” one side of which is convex and the other concave. A good example of the meniscus may be found in the old-fashioned watch-glass, before watchmakers took to flattening them, and watch-wearers were not ashamed to carry a “turnip,” in which there was room to spare for the works. If a section of such a glass were taken, it would assume the form of a half-moon. This, in fact, is the meaning of the term “meniscus,” which is a Greek word, signifying a little moon. If the same glass were solid, or even filled with water, it would form a “plano-convex” lens. Of course the outer curve of the meniscus must be larger than the inner curve, but in some cases the disproportion is very strongly marked, the outer curve being very large, and the inner curve very small. An example of such a meniscus may be seen in the human eye. If the reader will refer to the illustration on page 280, in which the structure of the eye is shown, he will see the meniscus lens in combination with the double convex. The former has already been explained, and the latter is formed by the vitreous humour which fills nearly the entire globe of the eye. Its larger curve is due to the form of the eyeball, and the smaller to the convex lens. LONG AND SHORT SIGHT. It has already been mentioned that the focus of a convex lens is shorter in proportion to its convexity, and that in consequence its magnifying power is increased. For example, the large glasses through which pictures are viewed are comparatively thin in proportion to their diameter, while the lenses employed for the highest powers of the microscope are scarcely larger than small shot, and nearly as globular. It naturally follows that any instrument to which a lens is adapted, whether it be microscope or telescope, must depend for its focus on the greater or less convexity of the lens in question. [Illustration: FRAME OF OWL’S EYE. LENS OF DITTO.] [Illustration: SHORT AND LONG SIGHTED EYES (WITH SPECTACLES).] Again taking as our example the human eye, we find that there are very few persons who from youth to age possess or preserve eyes which can read small type at a moderate distance, and can clearly define the outlines of distant objects. Nearly all people, even if in their youth they possess good sight, lose it as they grow older. They can discern distant objects well enough, but, when they come to reading, they are obliged to hold the book at arm’s length before they can distinguish the letters. This defect is caused by the insufficient convexity of the lens, so that the focus is thrown too far back, and it is corrected by wearing spectacles sufficiently convex to supply the deficiency in the lens of the eye.