Wundt was interested in sensations; and in explaining how sensations combined into perceptions, he indeed remained close to traditional associationism. In his book Principles of Physiological Psychology (1874/1904), Wundt expressed his belief that reaction time could supplement introspection as a technique for studying the elemental contents and activities of the mind. Friedrich Bessel performed the first reaction-time experiment to collect data that could be used to correct for individual differences in reaction times among those observing and reporting astronomical events. Others, such as the physiologist Helmholtz, used the method as well.
Franciscus Cornelius Donders. The famous Dutch physiologist, Franciscus Cornelius Donders (1818–1889), conducted an ingenious series of experiments involving reaction time by noting how long it took a subject to respond to a prede- termined stimulus (such as a light) with a predeter- mined response (such as pressing a button). Donders reasoned that by making the situation more com- plicated, he could measure the time required to per- form various mental acts.