Research in physiological optics retained throughout the nineteenth century a prominent role in philosophical and scientific descriptions of vision and the observer. The perceptual theories of the influential and versatile German scientist Hermann von Helmholtz, whom we shall encounter in this chapter and again later on, were heavily informed by his physiological studies. Yet Helmholtz's physiological researches were for him often a stepping stone for broader epistemological considerations. In this regard his work on vision was part of a general trend: to construct psychologically grounded theories of perception and knowledge (the two being virtually the same thing for an empiricist such as Helmholtz) that were by and large less concerned with how the organ of sight reacts to stimuli than with how sense data are processed by the mind of the observer. One immediate consequence of this discursive reorganization was the adamant rejection of the notion that a flawed organ of sight is the epicenter of misperception, confusion, and doubt. For Victorian epistemologists and philosophers of science such as John Stuart Mill, William Whewell, Herbert Spencer, and George Henry Lewes, the question of what happens when we misperceive is one that only tangentially concerns optics. Rather, they maintained, the problem has to do with inference, that is to say with the interpretation of sensation.

Ghosts, as we have seen, were regularly treated by nineteenth-century physiologists as illustrations of everything that can go wrong with or in the eye.