218 Chapter 14
devices helped him develop a “ sign theory ” that associated each such movement and its
muscular state with the attendant visual perceptions, no longer considered as realities in
themselves but as symbols of underlying physiological states and their external
correlates.^5
Here, as throughout his career, Helmholtz used his experimental findings to ground his
theoretical work.^6 Thus, his work on the mechanisms of vision led to his paper “ On the
Theory of Complex Colors ” (1852), in which he revived the three-color hypothesis of
Young and gave it new and fuller support from his own investigations.^7 This substantial
outpouring of specialized researches on many aspects of human vision finally led to his
massive Handbuch der physiologische Optik (Handbook of Physiological Optics , whose
first edition appeared in three parts during 1856 – 1866), a summa whose synthetic breadth
and systematic rigor put the whole field of physiological optics on a new plane of activity
by applying physical principles to anatomical structures.
In part, Helmholtz accomplished this by including a historical dimension in his work,
both to establish its sources and to make explicit its fundamental presuppositions. In the
midst of his experimental studies, he was constantly looking to the larger theoreticalFigure 14.1
Helmholtz ’ s myograph, used to measure the time required for nerve conduction in the thigh muscle of a frog
(1873).