138 CARNEGIE INSTITUTION 



Notwithstanding the reasons adduced (pp. 97 and 98), some as- 

 tronomers still think that we ought to restrict ourselves to the 

 brighter stars and those of considerable proper motion. 



We may now exclude the former from consideration, because, 

 for obvious reasons, a photographic Durchmusterung will do little 

 for the very brightest stars. They must be treated by instruments 

 c and d, and there will be little difference of opinion as to the desir- 

 ability of investigating as many of these stars as may be possible. 



The real question thus is : Has the time come to make a com- 

 plete Durchmusterung of parallax for the fainter stars (say 6.0 to 

 1 0.0) ? Or may we restrict ourselves to stars of sensible proper 

 motion only? 



It will be a relatively small undertaking to obtain the parallaxes 

 of the 200 stars of greatest proper motion (this is about the number 

 of stars with proper motion exceeding o".6, known at present, a 

 great part of which are bright ones). 



It seems not too much to expect that these will be observed else- 

 where. In fact, I think the greater part of them have already been 

 measured. For the rest, if necessary, they will be dealt with by the 

 instruments b, c, d. 



Setting aside also the consideration of these stars, therefore, it 

 remains to answer the question : What are we to do afterwards — 

 after the observing of the 200 stars of greatest proper motion ? 



The more important part of our aim must be to get a knowledge 

 of the distance of a great part at least of our nearest neighbors in 

 the universe, in order that we may begin by making a study of the 

 laws in their distribution and motions. Now, if from the very 

 beginning we exclude all the stars of which the proper motion at 

 right angles to the visual ray is small (and we virtually do this by 

 confining ourselves to stars of great proper motion), then we may 

 foresee at once that the finding out of any real law in the motions 

 will be impossible. Our aim will be defeated from the very outset. 



There are other considerations more amply set forth in the paper 

 quoted which must lead to the conclusion that for a study of the 

 construction of the stellar world we cannot escape the necessity of 

 making a Durchmusterung for parallax. It would be a noble task 

 for the observatory to be erected to take the lead in such an under- 

 taking. 



For a fairly fine climate I estimate that the two telescopes a 

 together would furnish a duplicate set of plates for the whole sky 

 from declination — 20 to declination — 90 in about eight to ten 

 years. 



