Life (March 1928, pages 143–145) entitled "What Deer Eat." The investigation of the food of deer was inaugurated in June 1926 in the Yosemite Valley by Professor A. W. Sampson of the Forest Service and this work was supplemented in 1927 by the studies of Joseph Dixon. With note book, pencil, binoculars, watch and camera ready for instant use the deer were followed, as quietly as possible, and the species of plants actually eaten were recorded. The identical plants upon which the deer fed were collected, dried and saved as herbarium specimens. This insured correct identification of any questionable plant and showed the extent and manner in which the plants were browsed. Examination of the stomach contents of deer shot in the open were used to check the results obtained in other ways. Dixon used the following methods to express the food preference of deer. The number of deer that browsed upon each species of plant was noted. The time or duration of each browsing was recorded. Then by multiplying the number of deer selecting any species of plant by the minutes spent in browsing he obtained what he designated as "deer minutes." Thus, if two deer browse on curly dock, each for a period of five minutes, the results would be totaled as, curly dock-10 deer minutes, when such observations were made daily for a period of two weeks, the investigators began to know what food was eaten by the deer in the Yosemite at that season of the year. To overcome the difficulties of observations made in thick vegetation a 17 inch f. 5.4 Ross telecentric lens and a reflecting camera were used, so that photographs of browsing deer were obtained of sufficient clearness and size to show what was being eaten. Dixon and Sampson found that the food of deer varied greatly with locality and season. Similar investigations might be used profitably in following the herds of caribou, as they migrate across the tundra at different seasons of the year, in order to collect data on which the management of such herds and the maintenance of the range plants might be based. This study of the Tundra Vegetation of Central Alaska is in part a contribution to that desirable end.

THE ATOM

By W. F. G. SWANN

IN ATTEMPTING a review of the story of modern atomic structure, it is appropriate that we first renew our acquaintance with a few of the principal actors in the play.

First we have the electron-the fundamental unit of negative charge a thing usually thought of until quite recently as a small spherical shell of negative electricity of such minuteness that if you should magnify the diameter of that shell to the size of a piece of small lead shot, that piece of lead shot would, on the same scale of magnification, become larger than the sun. The mass of the electron is so small that if you should magnify all masses so that the electron attained mass of two and a half grams, that two and a half grams would, on the same scale of magnification, become as heavy as the earth.

Then we have the "proton," the fundamental unit of positive charge a thing 1800 times as heavy as the electron, but 1800 times smaller in size, so that if you should magnify it to the size of a pin's head, that pin's head would, on the same scale of magnification, attain a diameter equal to the diameter of the earth's orbit around the sun.

Out of these two bricks, the proton and the electron, the attempt has been made to explain all the architecture of nature, and all that nature does. What then are the properties of these bricks whereby they may assume so great a responsibility? The properties which have been imputed to them are comparatively simple. When all is at rest, protons repel protons, electrons repel electrons, while protons and electrons attract each other, the forces in all cases varying according to the inverse square of the distance between the particles concerned, and acting in the line joining them.

We speak of a space around a charge as containing an electromagnetic field. The lines of electric force due to a charge are the lines along which a similar charge B, at rest, would be repelled by the charge on A. When A is at rest these lines travel out radially from it. When A is in motion with an unchanging velocity the directions of its lines of electric force are unaltered, and the intensity of the electric field is unaltered except for velocities comparable with that of light, velocities of the order 186,000 miles per second. The moving charge A is, however, accompanied by lines of magnetic force which, in the case of an unchanging velocity, form circles around its line of motion. These lines do nothing to another charge B so long as it is at rest. If it moves, however, they exert a force on it over and above that exerted by the electric field, a force perpendicular to the lines of magnetic force and to the direction of motion of the charge B. This force due to the magnetic field is the same force as we meet with in an electric motor where the wires carrying the current are set into motion by the magnetic field of the motor.

If an electric charge has its velocity changed suddenly, a sort of kink is produced in its lines of force, and this kink travels out along each line of force, its direction being practically perpendicular to the line of force. Theory also shows us that this transverse electric field as it is called is accompanied by a special magnetic field. If the charge moves backwards and forwards like a pendulum, these kinks are made first to one side and then to the other, and travel out into space after each other, in the form of a wave motion. A similar thing happens in the case of a charge moving in a circle. If a lot of elastic threads were tied to the walls of a room and their other ends were tied into a knot in the center of the room, and if the knot were then caused to describe a small circle, waves would travel out along each of the elastic cords and the situation would be closely analogous to what we should have in the case of the lines of force of an electron which was rotating in a circle.

The great electromagnetic theory invented by Maxwell on

the basis of the experimental researches of Faraday, tells us the detailed story of the electromagnetic fields of moving charges, and among other things we know that if a charge sends out waves of this kind into the surrounding space, those waves carry electromagnetic energy, and can only be created at the expense of a continual loss of energy of the charge responsible for them.

Thus, to cite an example, an electron can move around a proton in a circle as a planet goes around the sun, the attraction of the proton for the electron being balanced by the centrifugal force of the latter. Were it not for the electromagnetic radiation, the electron could go on moving in the same circle forever. On account of this radiation, however, it must lose velocity and start to fall into the proton by spiralling around in circles of ever decreasing radius. The laws of dynamics teach us, moreover, that the nearer an electron gets to the center of attraction the greater the number of times it will go around per second, so that the more closely packed will be the waves which it sends out into the surrounding space. Such a system would not therefore emit radiation of a constant wave length, but radiation of a wave length which changed continually.

Such then are the official properties of the proton and electron-all they are entitled to on account of their heritage as the offspring of Maxwell's great electromagnetic scheme of philosophy. As we might expect, we shall find them in difficulties occasionally in their ambitious task of explaining the universe, and shall see them stealing here and there an extra property to which they have no inherent right, but which they must have to carry on their business.

The problem of the atom has three primary aspects, the chemical, the radioactive, and the spectroscopic. Since it is around the third aspect that the wars have waged most hotly during recent years, and since it is the spectroscopic phenomena which have led to those developments which in the last few years have bid fair to revolutionize our thought in philosophy, I shall concentrate upon them almost ex

clusively, and shall make only passing reference to the others.

A characteristic feature of the atom is that when caused to emit light in a manner more or less unimpeded by its neighbors, it does so in the form of perfectly definite vibrations of frequencies characteristic of the atom. We used to think, and for many purposes we may continue to think of these vibrations as passing out in the form of waves in an allpervading æther. The greater the frequency of the vibrations, the smaller the lengths of the waves. The older theories had a most baffling task to account for the observed facts. Everything seemed to suggest that things should have been different from what they were.

The most alluring thought was of course to picture the atom as a little solar system, with the electrons rotating around a central nucleus of positive electricity as the planets rotate around the sun. We have already discussed the objection to this scheme. The emission of radiation by the atom would be accompanied by the spiralling of the electrons in towards the nucleus and the radiation would change its frequency continuously, so that instead of getting from the atom a number of discrete pure vibration frequencies, we should get a blurred composit of frequencies. So fundamental was this difficulty that the first attempts at a theory of the atom discarded completely the possibility of the planetary system and endeavored to think of the atom as a lot of electrons embedded in a relatively large sphere of positive electricity like plums in a jelly. The vibrations produced by electrons in this condition when disturbed from their positions of rest can be shown to be pure and nonblurred so that the difficulty in this respect was surmounted. It soon became evident, however, that the positive electricity could not exist in the atom in the form of a large sphere. For by measuring the deflections suffered by certain charged atoms in passing through thin sheets of matter it became clear that there existed in the atoms conditions which demanded that the positive electricity should be all collected into dimensions far smaller than those of the atoms them