318 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908, 



These hypotheses are the following : 



1. The mixture was assumed to be the same at greater and smaller 

 distances from the solar system. 



2. The same was done for different distances from the galaxy. 



3. The universe was assumed to be transparent, that is, it was 

 assumed that the absorption of light in space is zero. 



Can we get rid of these hypothetical elements ? 



I think we can, at least to a very great extent. 



As to the first. Our figure 4 already goes far in enabling us to 

 judge whether it is true or not, for evidently both our sixth and our 

 ninth shell give the nature of the mixture, at least of the stars of 

 absolute magnitude 3.5 to G.5. Therefore, as far as these stars are 

 concerned, we are able to see whether or not the mixture is the same 

 at the distance of G50 light years as it is at the distance of ITO light 

 years. Likewise the figure enables us to make the comparison in 

 other cases. As soon as we possess the necessary data for a longer 

 range of apparent magnitudes, say down to the fourteenth or fifteenth, 

 we shall be able to dispense to a very large extent with our first 

 hypothesis. 



As to the second, the possible variation of the mixture with the 

 distance from the Milky Way, it is largely only the question of treat- 

 ing the stars in different galactic latitudes separately. As far as I 

 can see there are no particular difficulties in the way of such a separate 

 treatment, at least not since the nature of certain anomalies in the 

 distribution of stellar motions has been elucidated. 



ABSORPTION or LIGHT IN SPACE. 



Last, not least, is the universe really absolutely transparent? 

 There are reasons which make this seem very doubtful. A couple 

 of years ago I obtained some evidence in the matter which shows 

 that the absorption of light in space, if it exists to an appreciable 

 amount, must at least be so small that over a distance of a hundred 

 light years not more than a few per cent of the light can be lost. 

 To determine so small an amount to within a small fraction of its 

 total value will be a difficult task indeed. Still we can even now see 

 definite ways, which, given the necessary data for very faint stars and 

 nebuLie, will probably enable us to overcome this last difficulty. 



COOPERATION FOR OBTAINING THE NECESSARY DATA FOR VERY FAINT STARS. 



This want of data for very faint stars, which in the present inves- 

 tigation makes itself felt at every step, has led a number of astron- 

 omers to concerted action. 



The express purpose of their cooperation is to collect data of every 

 kind for stars down to the faintest that can practically be reached. 



