534 PEASE— MEASUREMENT OF STAR DIAMETER. 



5.5 X lO"^ cm. and it is assumed for a Orionis that the value of 

 the wave-length is 5.75 X io~^ cm., a true value for which must be 

 found by direct experimental work. The value of b found for 

 a Orionis is 121 inches (±10 per cent.). The approximate value 

 then for the angular diameter of a Orionis is .047". The agree- 

 ment of this value with those obtained by calculation, which range 

 from .031" to .051", is striking. If there is a falling off of intensity 

 toward the limb, as in the case of the sun, Michelson finds this value 

 would be increased by 17 per cent. Several determinations of the 

 parallax have been made for a Orionis and from these its distance b 

 may be found from the expression 



2062657? ., 

 a = miles, 



TT 



where 7^ = 93,000,000 miles, the distance of the earth from the 

 sun, and tt is the value of the parallax in seconds of arc. Measures 

 of the parallax thus far obtained are: Adams, .013"; Yale, .032"; 

 .'Schlesinger, .016"; Yerkes, .022", the weighted mean of which is 

 ,020". From these values the distance is about 9.6 X 10^* miles. 

 -Knowing the distance and the angular value of the star, its linear 

 diameter is found to be 218 X 10° miles. This value is not a 

 definite figure but only an approximation ; but in any case it means 

 that the diameter of the star is several times the distance between 

 the earth and the sun and several hundred times the diameter of 

 the sun itself. 



The work is being continued until the half dozen stars which 

 calculations indicate as measurable with the twenty-foot beam have 

 been investigated. Most of these are stars having late type spectra. 

 In order to measure diameters of early type stars such as Sirius 

 and Procyon a much longer base is needed. For this work an 

 interferometer with mirror separations as great as 50 or 100 feet 

 has been discussed but it is felt the present instnmient should be 

 used to its limit and many data accumulated, particularly regarding 

 seeing conditions with the mirrors widely separated, before anything 

 definite is attempted in the way of a larger instrument. 



Mount Wilson Observatory, 

 Pasadena, Calif. 



