The empirical relations used in the estimation of spectroscopic parallax should provide material for examining the simple pressure effect, as they are derived from the ratio of two lines in the same spectrum. Unfortunately the line ratios actually in use were selected [Pg 146] because they were convenient to measure, and gave (empirically) consistent results, not for reasons of theoretical tractability. Fourteen line ratios are used, for example, by Harper and Young,[421] but only four of these consist of pairs of unblended lines with known series relations. It is only for such lines that a useful test of theory can be made. TABLE XXIII M = +7 +8.8 +9.2 +11.0 +13.2 +6 +13.3 +5 +3.5 +5.0 +7.4 +9.6 +4 +10.0 +10.8 +3 -1.8 +0.5 +3.8 +6.7 +2 +6.7 +7.0 +7.6 +10.0 +1 -7.2 -3.8 +0.2 +3.2 0 +3.5 +3.4 +3.2 +3.0 1 -12.9 -8.2 -3.2 -0.3 -2 +0.3 -0.2 -1.4 -3.7 -3 -12.1 -7.3 -3.8 -4 -2.8 -4.2 -5.8 -10.0 M = +6 -2.0 -1.4 +5 -1.5 -0.5 0.0 +1.0 +4 +1.2 +1.3 +3 +1.8 +3.3 +4.4 +4.5 +2 +5.0 +3.0 +1 +4.8 +6.5 +7.8 +7.6 0 +6.5 +4.7 -1 +8.6 +9.3 +11.0 +11.0 -2 +12.0 +8.6 +8.6 +8.2 M = +6 -18.1 +5 -3.7 -5.8 -9.6 -14.7 +4 -15.3 +3 -1.0 -3.3 -6.6 -11.0 +2 -12.4 +1 +1.7 -0.6 -3.7 -7.5 0 -9.4 -1 +4.5 +2.0 -0.9 -4.0 -2 -6.6 -3 +7.2 +4.7 +2.0 -0.2 M = +6 +7.4 +6.2 +5 +6.2 +4 +4.3 +3.0 +3 +3.0 +2 +1.3 0.0 +1 0.0 0 0.0 -2.0 1 +2.6 -2 +2.0 +0.7 -3 +5.2 [Pg 147] The preceding table contains a transcription of the reduction-curve material given by Harper and Young for the four pairs of lines mentioned. Tabulated quantities are the “step differences” for the classes at the heads of the columns, and the absolute magnitudes contained in the first column. Presumably the irregularities of the observed curves have been smoothed out in forming the reduction table, but the figures will certainly give an indication of the direction in which a given line is affected by absolute magnitude. The predicted effect of lowered pressure upon the lines involved is contained in the table that follows: Line Source Max. Effect of lowered pressure 4071 Fe (sub) weakened throughout 4077 Sr+ (ult) strengthened in , weakened in and 4215 Sr+ (ult) strengthened in , weakened in and 4247 Sc+ (?) strengthened throughout range 4250 Fe (sub) weakened throughout 4455 Ca (sub) weakened throughout 4494 Fe (sub) weakened throughout The predicted changes in the line ratios with lowered pressure are therefore as follows: