Henrietta Leavitt's classical Cepheid relation

The Leavitt & Pickering (1912) mean period-magnitude relation for SMC Cepheids exhibits correlated residuals relative to modern Gaia $B_G$ and Johnson $V$ photometry ($r_{\delta V\mathrm{/}\delta L12}\approx 0.76$). Therefore dispersion inherent to Leavitt’s photographic photometry relation is not solely random scatter (${\sigma }_{L12,B_G,V}=0.22\pm 0.03,0.25\pm 0.04,0.20\pm 0^m.03$), and is potentially consistent with the partial encoding of combined effects from the elongated SMC, classical Cepheid instability strip, differential reddening, etc. OGLE data indicate that $\gtrsim 870$ SMC variables identified by Leavitt (1908, Table 2) are classical Cepheids ($\gtrsim 90$ %), and the preliminary unphased Table 2 photometry likewise features correlated residuals relative to contemporary observations ($r_{\delta B_G\mathrm{/}\delta L08-T2}\approx 0.54$). Table 6 data in Leavitt (1908, $n=16$) define a highly linear Leavitt law over an extensive baseline ($r\approx -0.98$), but constitute a smaller sample than the later 1912 work ($n=25$).