356 Sir Frank Watson Dyson [April 29, 



determined only by the capacity of large telescopes to give measurable 

 spectra. In the table a comparison is given with unpublished results 

 at Greenwich obtained by the trigonometrical method : — 



Comparison of these results, obtained by entirely different methods, 

 shows the accuracy of 20 per cent, claimed for Mount Wilson, and 

 ±0*010" for Greenwich is reached. 



A third method which is employed extensively for determining- 

 stellar distances depends on the fact that the masses of stars lie 

 within very restricted limits. It is applicable only to double stars, 

 and depends on Kepler's third law, M + m = « 3 /T 2 > where M, m are 

 the masses, a is the mean distance between the components, and P 

 the period of a double star. When V is known aud M + m assumed, 

 a is found, and, further, as the cube root of M + m is involved, an 

 error in the assumed mass produces a much smaller error in the mean 

 distance. Now the angular mean distance is determined by direct 

 observation for all double stars the orbits of which can be calculated. 

 At the present time this amounts to more than 150. But it has been 

 shown by Hertzsprung and Russell that for double stars which have 

 completed too small a portion of their orbits for their periods to lie 

 known it is still possible to obtain their "hypothetical" parallax with 

 considerable probability. The method has been recently applied at 

 Greenwich to obtain the parallaxes of a large number of stars, and 

 the accordance with the results found by the trigonometrical and 

 spectroscopic methods is very satisfactory (see a paper in Monthly 

 Notices R.A.S., November, 1920, Vol. LXXXI. p. 2, by Messrs. 

 Jackson and Furner). 



I believe there is in preparation by American astronomers a 

 catalogue giving the parallaxes of 3000 stars, about half of which 

 have been determined by two at least of these three methods. We 

 may expect that in the course of a very few years the distances of all 

 stars visible to the naked eye in the northern hemisphere will have 



