Historical Background for Research in Subjective Visual Phenomena
There are sensations which do not correspond to anything outside the body. In so far as they imitate the qualities and forms of external things, they thereby often give rise to illusions, phantoms, or appearances with no corresponding reality. These can be referred to as subjective sensory phenomena.
— Purkinje (1819/1823a, pp. 3–4)
Subjective visual phenomena had a long history before Purkinje gave them that title, but he added greatly to the detail of their classification, description, and interpretation. The description of one's own sensations might seem the simplest of things to report on, but this is not the case. Helmholtz appreciated this only too well, and he commended Purkinje's special talent in this regard. Purkinje's doctoral dissertation, published in 1819, was entitled Contributions to the Knowledge of Vision in its Subjective Aspect. When it was reprinted in 1823 the title was extended to Observations and Experiments on the Physiology of the Senses. Contributions to the Knowledge of Vision in its Subjective Aspect. He commenced his research on visual phenomena because he had little access to equipment that would have