Before Einstein, the Kelvin-Helmholtz theory explained the Sun's energy through slow contraction. This theory estimated the Sun's lifespan as 20 to 500 million years.

THE following translation of the critical account given by von Helmholtz of the colour-theory of E. Hering, in the new edition of his Handbuch der Physiologischen Ohtik, commencing at page 376 ...

IN the course of a re-perusal of Helmholtz's “Ton-empfindungen,” it occurred to me to compare the theory of consonance and dissonance, there propounded, with the exposition of that theory ...