From the May 31, 1930, issue


The picture on the cover of this week’s SCIENCE NEWS-LETTER shows how an archaeologist masters the “human fly” trick when he must measure the stones that form the sloping walls of a pharaoh’s tomb. The scene is the famous pyramid at Meydum, Egypt, supposedly built by King Snefru. The Museum of the University of Pennsylvania is probing the pyramid’s secrets.


Because X-rays do not bend when they hit a solid object, but either pass straight through or stop dead, they have become one of the most important of the tools of modern physics, laying bare the secrets of crystals and showing how atoms are arranged in molecules.

The usefulness of these invisible radiations was discussed in a radio talk by Sir William H. Bragg, director of the Royal Institution of Great Britain and one of the world’s leading physicists. He was honored a few days ago by the presentation of the Franklin Medal of the Franklin Institute in Philadelphia. The talk was given over the Columbia Broadcasting System, under the auspices of Science Service.


The growth of plants can be seen with a new form of interferometer devised by Prof. K.W. Meissner of Frankfort, Germany.

The instrument is a modification of the interferometer invented by Prof. A.A. Michelson, of the University of Chicago, and used by him in his epoch-making experiments with light.

The interferometer is a device that permits the measurement of very tiny distances, far beyond the reach of the most powerful microscopes, by means of light waves. A beam of light from a lamp is separated into two rays at a lightly silvered glass plate, and each of the two beams is reflected from a mirror, the two being reflected back to the plate, where they reunite and fall into an observing telescope. When two such beams are properly superposed, they are capable of “interfering,” and we have the curious situation of light added to light giving darkness at certain points. For what one sees in the telescope is not a uniformly illuminated field, but a series of alternating bright and dark bands, or “interference fringes.”

If, now, one of the mirrors be slightly displaced, the fringe pattern moves to one side, and the distance it moves is a measure of the motion of the mirror. So sensitive is the method that it is readily possible to measure a displacement of the mirror of a millionth of an inch.

Prof. Meissner mounted the entire instrument vertically, thus bringing a whole new range of measurable phenomena within its scope. The movable mirror is carried by one arm of a trip-scales arrangement that permits a vertical motion of the mirror. The scales are very nearly balanced, and the mirror arm is allowed to rest very lightly on the stem of the plant whose rate of growth is to be measured. As the plant grows, it pushes up the movable mirror, and the interference bands in the telescope are seen to wander across the field. Simply counting the number that pass a given mark in a certain time gives the rate of growth, which is of the order of one hundred-thousandth of an inch per second for most plants, so that a single line would move more than its own width in a second.

Ether fumes are wafted over the plant almost immediately once the growth ceases; a mercury lamp, rich in ultraviolet rays, is switched on, and the rate of growth increases many fold. It is such investigations as these that the botanist Prof. Laibach is carrying out with the new instrument.

Prof. Meissner, in demonstrating his device before the Congress of Physicists and Mathematicians in Prague, pointed out, among other uses of the instrument, the measurement of crystal growth and the analysis of musical tones and vibrations.

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