We use the tapering effect of H \alpha / [ \ion N2 ] rotation curves of spiral galaxies first noted by Goad & Roberts ( 1981 ) to investigate the internal extinction in disks .
The scale length of exponential fits to the inner part of rotation curves depends strongly on the disk axial ratio .
Preliminary modelling of the effect implies substantial opacity of the central parts of disks at a wavelength of 0.66 \mu .
In addition , the average kinematic scale length of rotation curves , when corrected to face–on perspective , has a nearly constant value of about 1.7 h ^ { -1 } kpc , for all luminosity classes .
The interpretation of that effect , as the result of the increasing dominance of the baryonic mass in the inner parts of galaxies , yields a mean baryonic mass–to–light ratio in the I band \bar { \Upsilon } _ { I } = 2.7 h M _ { \odot } / L _ { \odot,I } , within the inner 1.7 h ^ { -1 } kpc of disks .