168 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1916. 



number of stars including this supposed limiting magnitude is 

 probably between one and two thousand millions. 



But why is it that there is a limit of numbers? Are we to suj)- 

 pose that there are no more stars, and that if our telescopes were 

 sufficiently powerful to perceive those of twenty-sixth magnitude 

 we could see all, little or big, that exist ? Or are we rather to sup- 

 pose that there is a limit of distance beyond which no star can be 

 seen, however bright, so that though myriads without limit may 

 exist, no single station in the universe is able to receive light from 

 those beyond this limiting distance? It seems probable that the 

 latter hypothesis is the true one, although astronomers would not 

 be unanimous in saying so. 



In recent years one bit after another of evidence has come out, 

 tending to show that there is a light-absorbing medium in space. 

 It is very rare. Dr. L. V. King has recently computed that the most 

 probable measures of its effects on star brightness would be satisfied 

 by assuming a density of the supposed absorbing medium in space 

 less than one-trillionth part of that of the air. But even at this 

 rate, space is so vast that the quantity of the supposed medium 

 within a sphere whose radius is the average distance of the nearest 

 star (a Centauri) is about 10,000 times the mass of the sun, which 

 is startling if true. 



There figures are of course very uncertain. But that there is in 

 space here a particle, there another, yonder a hydrogen molecule, 

 beyond still others, and that in the well-nigh endless path extend- 

 ing to stars of the twenty-sixth magnitude, whose light traveling 

 186,000 miles per second takes tens of thousands of years to travel 

 to us, there would be found enough such particles to bar the doors 

 of light, as a fog shuts out the sun — this seems reasonable. 



