152 STELLAR EVOLUTION. 



with ju8t such a lens 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 l)e 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 (PI. II). This is due to the fact that the 

 earth is rotating upon its axis at such a rate as to cause ever}" 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 b}- 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 neces- 

 sary to mount the camera in such a wa}'^ that it can 1)e 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 i()-inch Yerkes telescope. 



But the scale of the photogra[)hs obtained with such a camera differs 

 in marked degree from that of the photographs furnished b}^ the tele- 

 scope. Here, for example, is a region of the Milky Way, photographed 

 by Professor Barnard with one of the old-fashioned lenses formerly 

 emploj^ed in portrait galleries (PI. III). 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 com- 

 mon to this area. You will notice in the midst of this star cloud a 

 little cluster of stars, here so denseh' 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 pur- 

 pose 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 6 inches in diameter, 

 of 81 inches focal length. The great telescope has a lens 40 inches in 

 diameter, of 64 feet focal length. Thus the scale of the photographs 

 made with the telescoi)e is about twenty-five times that of the photo- 

 graphs made with the portrait lens. The portrait lens covers a large 

 area of the sky on a very small scale, while the tield 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 as illustrated by 



