Answer: The psychologist who conducted this experiment was Wilhelm Wundt, and he used a method called introspection.
Explanation: Psychologist Wundt first began with the experimental application of introspection. Introspection as a method existed even before Wundt and was used by philosophers to study ways and procedures of how new ideas are generated. What Wundt first started with was the application of this method as a way of looking inward, into one's own thoughts and feelings, for experimental purposes.
So introspection is the concentration and examination of one's own thoughts, that is, one's own mental state. It is a conscious peek into one's mental state by tracking one's thoughts and feelings fully consciously, thus gathering information about conscious experiences. In other words, it is a process of self-examination where the experimenter perceives his own thoughts, analyses them and reports on them.