TABLE IX Residuals in m_{T} + 5 log d as a Function of Orientation ===============+=============+===============+============ Type | Round | Elliptical | Edge-On ---------------+-------------+---------------+------------ Sa | –0.02 (13) | –0.27 (13) | +0.57 (23) Sb | .77 (24) | .0 (35) | 1.71 (11) Sc | –0.08 (35) | –0.13 (57) | +0.66 (22) All S | –0.26 (72) | –0.11 (105) | +0.83 (56) SBa | 0.0 (10) | –0.30 (7) | +0.31 (8) SBb | – .16 (10) | + .07 (6) | SBc | +0.19 (9) | –0.50 (4) | +0.32 (2) All SB | +0.01 (29) | –0.21 (17) | +0.31 (10) All spirals | –0.22 (101) | –0.13 (122) | +0.73 (66) ---------------+-------------+---------------+------------ Among the elliptical nebulae it is observed that the nuclei are sharp and distinct and that the color distribution is uniform over the images. This indicates that there is no appreciable absorption, either general or selective, and hence that the luminosity of the projected image represents the total luminosity of the nebula, regardless of the orientation. If the observed classes were pure, that is, if the apparent ellipticities were the actual ellipticities, formula (1) could be written $C_{e} = m_{T} + 5 \log b - 5 \log (1 - e),$ (3) where b is the minor diameter in minutes of arc and e is the ellipticity. The term m_{T} + 5 log b is observed to be constant for a given type. If it were constant for all elliptical nebulae, then the term C_{e} + 5 log (1 – e) would be constant also. On this assumption, $C_{e} + 5 \log (1 - e) = C_{0},$ where C_{0} is the value of C for the pure class E0. Hence $C_{e} - C_{0} = -5 \log (1 - e),$ (4) a relation which can be tested by the observations. An analysis of the material indicates that this is actually the case, and hence that among the elliptical nebulae in general, the minor diameter determines the total luminosity, at least to a first approximation.[18] The observed values of C vary with the class, as is seen in Table VII and Figure 6, but, excepting that for E7, they are too large because of the mixture of later types of nebulae among those of a given observed class. It is possible, however, to calculate the values of C_{e} – C_{0} for the pure classes and then to make approximate corrections for the observed mixtures on the assumption that the nebulae of any given actual ellipticity are oriented at random. In this manner, mean theoretical values can be compared with the observed values. The comparisons are shown in Table XII in the form C_{7} – C_{e}, because E7 is the only observed class that can be considered as pure. The significance of the table will be discussed later.