Fechner's Law, is a formula that describes the proportional relationship between physical stimuli and perceived magnitude. In other words, how we perceive the magnitude of physical processes acting upon our sensory organs and nervous system. Weber's law extends this into relative terms, where it describes the minimal magnitude change for an external stimulus to be perceived as different from another (just noticeable difference, JND.) This can be how much brighter a light source, louder a sound, saltier a broth, longer a line, etc. has to be to notice a difference

I think the day marks the anniversary when Fechner, according to Wikipedia:

"Whilst lying in bed, Fechner had an insight into the relationship between mental sensations and material sensations. This insight proved to be significant in the development of psychology as there was now a quantitative relationship between the mental and physical worlds."