As stated here on Wikipedia: 1
Weber's law states that the-just-noticeable-difference (JND) of an intensity of a stimuli divided by the intensity of that stimuli is always constant.
... where $I$ here means the physical intensity of sound, light and so on. $\Delta(I)$ is the-just-noticeable-difference.
Then came Fechner and assumed that the-just-noticeable-difference "in sensation" of a stimuli is constant as well, hence: $\Delta(I)/I = Constant = \Delta(S)$
$\Delta(S)$ stands for the-just-noticeable-difference "in sensation."
On what basis was Fechner justified in assuming that the-just-noticeable-difference in sensation is constant?