$\star$ Corrections of extragalactic extinction

However, when one seeks to make the photometry of objects which one knows the color a priori , it is possible to determine the excess of color by a measurement of the color of the object. Provide with the excess of color, one applies the same extinction law as for the Galaxy. The problem of this method is that it supposes that the extinction law is universal.

Finallemnt, nothing ensures that the chemical evolution does not deteriorate these laws. Moreover, there are some clouds of dust in our Galaxy where this law is put at fault, ${ \cal R}$ takes in this case a value different from the canonical value.