STELLAR EVOLUTION. 295 



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 astro- 

 nomical 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 in- 

 strument 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 photo- 

 graphed 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 in- 

 strument 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 



