“…We now know that this conclusion was correct, although Shain's value: of 28 seconds for the difference between the periods was excessive. Later evaluations ( 3 , 9,10,13) placed the difference closer to 10 or 11 seconds, and a recent redetermination by our group, using data which now extend over a decade, has given a figure of 9"55"'29'.35 for the rotation period of the radio sources (5). Since J. N. Douglas and H. J. Smith recently derived, by an independent method, a value which differs from ours by only 0.02 second (II), the period now seems to be established with high precision.…”