Note 5 — Chapter II · SENSATION IN GENERAL.
Reading edition
In other words, S standing for the sensation in general, and d for its noticeable increment, we have the equation dS = const. The increment of stimulus which produces dS (call it dR) meanwhile varies. Fechner calls it the 'differential threshold'; and as its relative value to R is always the same, we have the equation dR/R = const.