Page:Popular Science Monthly Volume 25.djvu/442

430 of measles, scarlet fever, diphtheria, whooping-cough, or from any one of the depressing diseases of childhood, unable to study as they did before the attack. A weak magnifying spectacle, by helping the muscles to do their work, will enable such children to continue their studies till tonics daily administered restore the needful strength to the enfeebled muscles. The foregoing statements are based upon perfect eyes. Unfortunately, the eyeball, with the many other features, has not always the perfection of symmetry. Near-sighted long eyes and over-sighted flat eyes are the common deviations from the standard shape. In the near-sighted eye, called myopic, the eye is so long from front to back that the lens is too far from the retina. The result is, that rays of light from a distant object come to a focus, and have begun to diverge when they reach the retina, so that the image formed is blurred. The second deviation in the form of the eye is called hyperopia. This is a flat eye, a very common form in children. It is a congenital defect, in which the crystalline lens is located so near the retina that light, passing into the eye, is stopped by the retina before it comes to a focus. This must also produce an ill-defined picture. Unfortunately, faulty eyes, which give out under use, do not appear differently from perfectly shaped ones. The flattening, or the elongation, is not in the exposed cornea. It is usually at the expense of the inner half of the eyeball, hid away in the socket. If children, either by inheritance or acquisition, have misshaped eyes, so that they can not see objects clearly through the usual range of distances, what can be the propriety of allowing them to go through life as if in a constant fog, when a properly selected glass clears up the mist, and enables them to see as others do?

Fresh-Water Pearls.—The cultivation of the pearls of fresh-water mussels has become an industry of considerable importance in Saxony and other parts of Germany. The pearls are generally inferior to those of the genuine pearl-oysters, but occasionally a gem of real excellence is produced. Some very fine settings of such were exhibited at the Exposition in Berlin. The Venetians carried on this branch of trade to a considerable extent during the middle ages, and controlled it till 1621, when the Elector of Saxony also undertook it, at the suggestion of Moritz Schmirler, a draper of Oelsnitz, and appointed Schmirler "first pearl-fisher." Schmirler was succeeded on his death by his son, and the business has continued in the family to the present day, under the superintendcncy of the forestry department, which has also to do with the waters of the region. The pearl-hunting is carried on in the spring, as soon as the water is warm enough to wade in for hours continuously. The mussels are examined by means of an instrument, by which the shells can be opened enough to see what is within them without hurting the mollusks. If they contain well-developed pearls, they are sacrificed; if not, they are returned to the beds. The same beds are not usually gone over again for several years. Experiments made in the Elster, in the artificial production of pearls, have not met with much success. A wound in the mouth of the mollusk will lead to the deposition of the calcareous matter, but it is uncertain whether it will be of common shell-matter or of pearl—and upon this all the value of the operation depends. In the Dutch East Indies, the formation of pearls in the pearl-oyster is sometimes provoked by inserting a grain of sand within the shell. A considerable business is done at Adorf in the manufacture of articles of fancy from the nacre of mussels.

Geological Survey of Palestine.—Professor Hull has just made a successful geological survey of Palestine, preparatory to the construction of a geological map of the country. He has traced the ancient margin of the Gulfs of Suez and Akabah to a height of two hundred feet above their present level, so as to show that the country has been submerged to that extent and has been gradually rising; and he believes that at the time of the Exodus a continuous connection existed between the Red Sea and the Mediterranean. The Dead Sea appears to have formerly stood at a height of fourteen hundred feet above its present level, or about one hundred and fifty feet above the level of the Mediterranean. Evidences of a chain of ancient lakes have been found in