*Effects of Red-Shifts on Apparent Luminosity*

The first step in the venture is the calculation of the proper corrections which must be applied to the measured apparent luminosities before the distances can be estimated. There are two possible corrections, the reasons for which are as follows. The apparent luminosity of a nebula is measured by the rate at which the radiant energy reaches the observer. The observer intercepts light-quanta, each carrying a certain amount of energy. A change in either the rate of arrival of the quanta, or in the individual energies they carry, will alter the measured luminosity. These considerations introduce two possible corrections, either or both of which must be applied according to circumstances.

The rate of arrival, the number of quanta which the
observer intercepts each second, is necessarily reduced
by recession of the nebulae. The appropriate correction, known as the
*recession factor*, must be applied if,
and only if, red-shifts are velocity-shifts. Here, in
principle, lies the empirical test of the interpretation
of red-shifts, which will be discussed later in considerable detail.