178 
ASTRONOMY: F. G. PEASE 
Proc. N. A. S. 
For j8 Geminorum (Pollux) fringes have been observed at separations 
of 14.5, 16, and 19 feet, the visibility being definitely reduced for these 
distances of the mirrors, although at 19 feet it is still considerable. A 
longer beam will therefore be required for the measurement of the star's 
diameter. 
For a Cetione observation at 13 feet in variable seeing showed a re- 
duction in visibility of the fringes when compared with that of the check 
star. 
For a Bootis (Arcturus) observations in February and March, with 
poor seeing, indicated a decided reduction in the visibility of the fringes 
with increasing separation of the mirrors. On April 15, a series of meas- 
ures was made under conditions which were excellent, as indicated by the 
strong contrast of the zero fringes and by diffraction rings surrounding 
the central disk of the star images. 
At 16 feet separation of the mirrors the visibility of the fringes was 
estimated at 20 to 25 per cent of the zero fringes; at 17.5 feet, 15-20 per 
cent, certainly less conspicuous than at 16 feet. At 19.0 feet the estimate 
was 5 per cent, the contrast being very flat, but the fringes were picked 
up with certainty as the path difference was varied with the optical wedge. 
At 18.25 feet, the result was 15 per cent, certainly greater than at 19.0 
feet; and finally at 19.5 feet no fringes whatever could be observed from 
the interferometer mirrors. There was no bright star in the immediate 
neighborhood that could be used as a check, but the instrument was known 
to be in adjustment for the final measure, from the certainty with which 
the previous settings had been made. 
The separation of the mirrors for disappearance of the fringes may there- 
fore be placed at 19.5 =±= 0.5 feet, although observations with a longer beam 
with which the second maximum could be observed, might possibly in- 
crease this value a trifle. By disregarding any possible darkening at the 
limb, the result will serve to indicate the maximum diameter of the star. 
The spectrum of a Bootis is Ko; its approximate effective wave-length 
may therefore be taken at 5.6 x 10 ~ 5 cm., about halfway between that of 
the sun (Go) and that assumed for a Orionis (Ma). 
With this value of X and a distance of 19.5 feet (594 cm.), the angular 
diameter of a Bootis is 0.0237. It is interesting to note the close agree- 
ment of this figure with the estimated values of Bddington (0.020) and 
Russell (0.019), and with Hertzsprung's value (0.026), calculated on 
the basis of the measured diameter of a Orionis (0.045). 
Results for the parallax of a Bootis are as follows: Yale, 0.074; Flint, 
0.095; Yerkes, 0. 100; Adams, spectroscopic, 0.158; the weighted mean 
is 0. 116. The distance of Arcturus is therefore of the order of 1.65 x 10 14 
miles (2.65 x 10 14 km.), and its linear diameter 19 x 10 6 miles (30.6 x 10 6 km.), 
or about 22 times the diameter of the sun. 
