sentences:  Lassen's work, completed two years prior to Appleton and five years prior to Hartree, included a more thorough treatment of collisional plasma; but, published only in German, it has not been widely read in the English speaking world of radio physics.Further, regarding the derivation by Appleton, it was noted in the historical study by Gilmore that Wilhelm Altar (while working with Appleton) first calculated the dispersion relation in 1926.The dispersion relation can be written as an expression for the frequency (squared), but it is also common to write it as an expression for the index of refraction