1908] on Recent Researches in the Structure of the Universe. 315 



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

 in the distribution of stellar motions has been elucidated. 



Absorption of Light ix 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 

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



Co-OPEKATIOX FOR OBTAINING THE NECESSARY DaTA FOR 



Very Faint Stars. 



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

 investigation, makes itself felt at every step, has led a number of 

 astronomers to concerted action. 



The express purpose of their co-operation is to collect data of 

 every kind for stars down to the faintest that can practically be 

 reached. As complete observation and treatment of these number- 

 less stars is out of the question, the plan is confined to a set of 

 samples distributed over the whole of the sky. 



Conclusion. 



If, at the end of this lecture, somebody summarises what has been 

 discussed by saying that the results about the structure of the 

 universe are still very limited and not yet free from hypothetical 

 elements, I feel little inclined to contradict him. But I would 

 answ^er him by summing up in another way, viz. : — 



Methods are not wanting which, given the necessary observational 

 data obtainable in a moderate time, may lead us to a true, be it 

 provisionally still not very detailed, insight into the real distribution 

 of stars in space. 



I think this time need not exceed some fifteen years. They to 

 whom such a time may still seem somewhat long, may be reminded 

 of the fact that that time will be elapsed, that we shall have jSnished 

 our work, before any but a very few of our nearest neighbours in 

 space can be aw^are of the fact that w^e have begun. 



[J.C.K.] 



