I912-] DEPTH OF THE MILKY WAY. 17 



two million light-years, and it is very probable that we can penetrate 

 to a depth of about five million light-years. A modern silver-on- 

 glass reflector of twelve feet aperture would give about six times 

 as much light as the 6o-inch reflector at Pasadena, and with this 

 gain of two magnitudes in light power it is probable that we could 

 penetrate into space at least twice this distance (theoretically 2.512 

 is the factor) or to a depth from which the light zuould take ten 

 million years to reach the earth. 



At the present time a 12-foot reflector is possible, and the depth 

 to which we can penetrate is simply a question of telescopic power, 

 which can be vastly but not indefinitely increased. And this is true 

 in spite of the extinction of light by cosmical dust in space. There 

 is a limit to the distance to which any given telescope can penetrate, 

 but it increases steadily with the aperture, since the only question 

 involved is one of enormous light grasp. 



It is to be hoped that a telescope of not less than 12 feet aperture 

 may be built for use on the Milky Way. With such a giant instru- 

 ment discoveries of the highest order might confidently be antici- 

 pated. A modern-- expansion of our views of the sidereal universe 

 analogous to that which marked the great epoch of Herschel would 

 follow, with the most beneficial effects upon every branch of astro- 

 nomical science. Recent developments in many lines show that the 

 epoch of great discoveries has not passed, but is in fact just begin- 

 ning : and the estimates here laid down, as to the depth and magnifi- 

 cent extent of the Milky Way, convey to us but a dim outline of the 

 discoveries which await the builders of the giant telescopes of the 

 future. In this great advance America may naturally be expected 

 to take the leading part. 



Starlight, on Loutre, 



Montgomery City, Missouri, 

 November 4, 191 1. 



years. The view adopted in my " Researches," Vol. II., 1910, however, is 

 much more probable, since it gives continuity to the various types of bodies 

 observed to constitute the sidereal universe. Note added Dec. 16, 1911. 



PROG. AMER. PHIL. SOC, LI. 203 B, PRINTED MARCH 16, I9I2. 



