- and
-stars. It would be a strange coincidence if this shift, to which all the evidence points as arising from a common origin, should be manifested just in these cases which have been made the object of an investigation.
(b) The general shift towards the red might be ascribed to pressure effects at the surfaces of the stars or to the presence of other lines which lie on the red side of the main lines, but which are very weak or even absent in the comparison spectrum of the sun. A detailed knowledge of conditions on the surfaces of stellar bodies could alone give a decision on this point.
2. It is only possible to prove that the shift
is not due to a radial velocity if one can measure the ordinary Doppler effect arising from the radial velocity separately. Let us consider a single
-star or group of