Vol. 7, 1921 
ASTRONOMY: MICHELSON AND PEASE 
145 
which appear as straight lines, alternately dark and light, perpendicular to 
the line joining the mirrors. 
As the outer mirrors are separated, the visibility of the zero fringes re- 
mains constant, while that of the interferometer fringes gradually de- 
creases until a point is reached where the latter vanish ; as the mirrors are 
further separated the interference fringes should reappear, approach a 
secondary maximum, and again disappear, this phenomenon being re- 
peated indefinitely. 
We are interested chiefly in the first disappearance. The distance be- 
tween the mirrors corresponding to extinction is the observed quantity re- 
quired to determine the angular diameter of the star. Thus far it has been 
necessary to move the mirrors by steps, as screws are only now being in- 
stalled to maintain the mirrors at equal distances. 
The visibility of fringes obtained with the interferometer pointed on 
Vega in August, 1920, with the mirrors separated 18 feet (5.5 m.), was fully 
as great as with the mirrors 6 feet (1.83) apart, thus indicating that atmos- 
pheric conditions will easily permit the use of even greater separations. 
Calculations of stellar diameters based on estimates of surface bright- 
ness made by Eddington, Russell, and Shapley, indicated that a Orionis, 
because of its relatively large diameter, would be a promising object to 
attempt to measure with the 20-foot interferometer. Merrill first ex- 
amined the star with the apparatus used by Anderson 2 in the measurement 
of Capella and found a definite decrease in visibility for the maximum sep- 
aration of the slits (100 inches). This was true for all position angles, thus 
indicating that the star was not a binary. 
On December 13, 1920, the interferometer was tested by Pease on /3 
Persei and then on y Orionis. Both stars are known to have diameters 
much smaller than can be measured with this instrument. When adjusted 
with the mirrors separated 121 inches (229 cm.), both the zero and the 
interferometer fringes appeared in the eyepiece. When the instrument 
was directed to a Orionis the interferometer fringes could not be found. 
a Canis Minoris was then observed and the interferometer fringes were 
easily seen thus indicating that the instrument was still in adjustment. It 
is thus clear that the disappearance of the fringes for a Orionis was real and 
not due to any disturbance of the mirrors, for the seeing was good and any 
flexure changes involve only a very slight shift of the compensating wedge 
to bring the fringes into view again. 
On December 14 and the nights following, the seeing was poor and as the 
visibility of the zero fringes was decidedly lower than on the night of the 
13th, no attempt was made to work on a Orionis. With a mirror sep- 
aration of 13 feet attention was directed to a Ceti, a Tauri and (3 Gemin- 
orum. The zero fringes were seen in every case, though much reduced in 
visibility, but as the interferometer fringes could not be seen at all except 
at intervals of better seeing, it is presumed that there was an actual de- 
