as is found in the possession of thousands of amateur photographers. If we take an ordinary camera and point it on a clear night toward the north pole, it will be found after an exposure of one or two hours that the stars which lie near the pole have drawn arcs of circles upon the plate (Fig. 2). This is due to the fact that the earth is rotating upon its axis at such a rate as to cause every star in the sky to appear to travel through a complete circle once in twenty-four hours. The nearer the star to the pole the smaller does this circle become. As we move away from the pole we find the curvature of the star trails growing less and less, until at the equator they appear as straight lines.
Just such photographs as these are frequently employed in astronomical investigations; e. g., for the purpose of recording variations in a star's brightness, which would be shown on the plate by changes in the brightness of the trail. But for most purposes it is desirable to have photographs of stars in which they are represented as points of light rather than as lines. To obtain such photographs it is necessary to mount the camera in such a way that it can be turned about an axis parallel to the earth's axis once in twenty-four hours. A camera so mounted becomes an equatorial photographic telescope, differing in no important respect save in the construction of its lens from an instrument like the 40-inch Yerkes telescope.
But the scale of the photographs obtained with such a camera differs in marked degree from that of the photographs furnished by the telescope. Here, for example, is a region of the Milky Way photographed by Professor Barnard with one of the old-fashioned lenses formerly employed in portrait galleries (Fig. 3). Such a picture as this is of the greatest service in all studies of the structure of the Milky Way, for it brings before us at a single glance an immense region of the sky, thus permitting us to trace the general features which are common to this area. You will notice in the midst of this star cloud a little cluster of stars, here so densely packed together that no details of the cluster can be distinguished. If our investigations required us to single out some individual star in the cluster, perhaps for the purpose of analyzing its light, it is evident that the portrait lens would prove inadequate for our purpose. It is in such a case as this that an instrument like the 40-inch telescope comes into play. The camera with which this photograph was taken has a lens six inches in diameter, of thirty-one inches focal length. The great telescope has a lens forty inches in diameter, of sixty-four feet focal length. Thus the scale of the photographs made with the telescope is about twenty-five times that of the photographs made with the portrait lens. The portrait lens covers a large area of the sky on a very small scale, while the field of the telescope is limited to a small region, which is depicted on a large scale. Let us see the difference between the two instruments