Science Siftings

New Zealand Tablet, 30 July 1908, Page 35

Science Siftings

BY 'VOLT

How a. Tree Grows". Both -earth and air. are required for the grow.th of a plant tree. The roots absorb .moisture from the soil, which, in the form of a watery fluid called common sap, rises, through the fibres of the last deposited annular ring, traversing all the branches and leaf staiks until it reaches the leaves; there it undergoes a change " by the absorption of carbonic acid from the air. It then travels downward again in the* form of proper sap, just underneath the bark, which is' expanded" by the accession of moisture, and in the cavity so formed a new layer of material is deposited which" gradually hardens and forms' a new annular ring. - And so, from absorbing the moisture and minerals of the soil and the carbonic acid of the air the tree goes on until it finishes its cycle and dies. Largest Telescope in the World to be Made.

At the present writing the famous forty ? inch Yerkes teles? cope at Wjlliams Bay, Wis., is the most powerful of its ..kind in existence, but "the eagerness of men to know more about the solar system has prompted the building of a, still greater instrument of the reflecting type. John D. Hooker, of Los Angeles, California, has provided the funds for the construction of a" reflecting telescope, ■of which the mirror will be loom in diameter and the focal length fifty feet. This instrument is intended .for the use of the Solar Observatory of the Carnegie Institution .on Mount Wilson. . The enormous stride in the erection of such a telescope will be best comprehended if its dimensions are compared with the largest instruments of the kind completed/ Hitherto sixty inches has been the limit of diameter of a silverglass mirror. In the . Ritchie sixty-inch mirror, the mass of glass of which the mirror is composed is eight inches thick and weighs one ton. In the new telescope the mirror will have glass thirteen inches thick and the mass will weigh four tons and a half. About four years will, be required for the making of this telescope. and another year for the mounting of it. How much nearer if will bring the stars, and" how much more detail in the planets it will give mankind, remain to be demonstrated.

Now and then mirrors of a curious kind are seen in Europe. They are called ' magic mirrors,' and are of Japanese origin, made, not of glass silvered, but of cast bronze, polished on the face, and bearing on the back raised patterns, inscriptions', symbolical designs, crests, or pictures. When exposed to a bright beam of light from the sun or from an electric lamp,- they reflect in the light from their polished face the image "of the pattern on their backs. This is a purely optical property, and has, of course, nothing in common with the fortune-telling magic crys-. tals of the astrologer or the alleged magic mirrors of necromancy, yet it long puzzled the scientific optician, and even now is little known or believed. The researches of .various scientific men have established the fact that the phenomenon is due to very minute differences of curvature in the polished face, differences so minute that they do not affect the ordinary use of the mirror as a looking-glass, and that can be detected only by delicate optical tests. The only remaining mystery has been as to how these delicate differences of- curvature -were produced in exact correspondence to the pattern on- the back. The makers themselves are often in ignorance of the magic property and do not know which of their mirrors possess it and which do not. The mirrors are cast in moulds and afterwards

polished by hand, and it is held by scientific men that the difference of curvature is caused by the metal- yielding unequally under that pressure of the .tools used in scraping and polishing, the thin parts naturally bending more than the thick. This" accounts for the mirrors becoming magic-