Visual Education (Jan 1923-Dec 1924)

February, 19 2 3 45 from view and can no longer be found in its favorite haunts in its elliptical orbit, then its place is probably taken by a swarm of meteorites, and if the earth passes through some part of this swarm a meteor shower ensues. Wonderful showers of this character were observed at intervals of thirty-three years — within a day or two of November 13 of the years 1799, 1833 and 1866; but the photographic plate was not ready for them. These are known as apparitions of the Leonid swarm, so-called because the direction in space from which their path intersects the earth's orbit is that of the constellation Leo. Their appearance has been traced back a thousand years in the ancient records. On November 15, 1899, the astronomers lay in wait for them with a regular artillery of cameras ; but the planets Jupiter and Saturn had given a gravitational tweak to the swarm, so that its orbit no longer crossed that of the earth, and our preparations were in vain as there was no shower of any consequence. Meteors are often caught darting across a plate during an exposure on some field of stars, as is illustrated in the photographs herewith. By photographing these shooting stars, during some of the smaller showers, with identical cameras at stations a few miles apart, it has been possible to determine their distance from the earth. Doubtless every one knows that they are only luminous as a result of friction in the earth's atmosphere, and this heat reduces most of them to dust before they strike the earth. Portraits of Planets The planets also may be studied with great advantage by photography, but with very different instruments from those used for comets. For extended objects or for large areas of the sky a short telescope is employed, generally from thirty to fifty inches in length and with an aperture of eight or more inches. For the planets a telescope of long focus is required, and, in order to PHOTOGRAPHS OF THE PLANET MARS Made by E. E. Barnard with tile 40-inch telesccpe of the Yerkes Observatory The planet is only 4,200 miles in diameter — a little more than half that of the earth. It has definite markings, as may be seen on these pictures, but we cannot positively find what they represent. The light orange portions may be desert areas, and the darker portions may be regions covered with vegetation. In order to take advantage of the instant of steady definition it is necessary to take several pictures on a plate. This photograph combines, in three vertical groups, pictures taken at an interval of a few seconds from each other, but the vertical group in the middle was taken one hour after the group on the right, and twenty-two minutes before the group on the left. Therefore, the group on the left was photographed one hour and twenty-two minutes later than the group on the right, and the erader will easily see that during that interval the planet has rotated. The south polar cap, probably of snow, is conspicuous near the top of each picture. get short exposures, its aperture must also be large. A secondary magnifier, something like an eyepiece, is also employed for the planets, because it is necessary to avoid a very minute image which would be distorted by the granular nature of the plate. The group of pictures of Mars given herewith shows the planet to be rotating while you wait, as is fully explained in the legend. These photographs were taken by Professor Barnard with the fortyinch telescope of the Yerkes Observatory. The Camera as Detective Photography has been very helpful in tracking lost asteroids, and it is responsible for the discovery of hundreds of new ones. These little bodies, generally only a few miles in diameter, perhaps chips from a planet which exploded — not once, but several times — move about in a swarm of orbits between Mars and Jupiter. Most of them do not get very far away from, the ecliptic, which is the plane of the earth's motion around the sun and near which the orbits of all the large planets are found. With an exposure of an hour or two, when a photograph is being taken of the sky in this region and when the telescope has been carefully guided during the exposure, the images of the stars will be small, round dots ; but if there happens to be an asteroid in the field it will make itself known by its motion during the exposure, thus leaving a little streak instead of a dot. Sometimes several of these streaks have been found on the same plate. If computations show that they do not correspond to any known asteroid, then these new asteroids are added to the list of nearly a thousand already discovered. Of course, the astronomers follow the object on successive nights until enough data are obtained for predicting its future motion under the law of gravitation ; then its orbit is known, and until Jupiter perturbs it by his attraction the place of the body can be predicted for any time in the future. The first known asteroid to be caught on a photographic plate was