Jiři Hoskovec
CSc. Psychologist, Historian of Science

Moskevská 32
Praha 10 101 00
Czech Republic

"It is an imperative belief of the natural scientist that each and every modification of a subjective state in the sphere of the sense's corresponds to an objective state" (Purkinje, 1819/1823, p. 92).

"In the quest to achieve his vision Jan Evangelista Purkinje or Purkyne (1787-1869) left his mark throughout the body. There are Purkinje cells in the brain, Purkinje fibers around the heart, Purkinje images are reflected from the optical surfaces of the eye, a Purkinje tree (the shadows of the retinal blood vessels) can be rendered visible, and at dawn and dusk we can experience the Purkinje shift. As a medical student he investigated subjective visual phenomena in part because he did not have access to any physiological apparatus, but also because he believed that visual illusions revealed visual truths. Purkinje's interests in vision were stimulated by reading Goethe's (1810) Our Farbenlehre (Theory of colors) as a medical student (Griisser, 1984; Kruta, 1966).

Goethe's theory was founded on phenomenological description of perceptual experience and he rejected the physicalism of Newton's (1704) and Young's (1802) theories. Goethe championed the alternative approach based on color experience rather than color mixing. Newton had stated that "the Rays to speak properly are not colored" (1704, p. 90), thus accepting the subjective dimension in color vision, but he did not subordinate the physics of light to the philosophy of sight in the manner of Goethe. One of Goethe's greatest difficulties was reconciling the purity of the perception of white light with the conception of its compound nature. However, he was able to enlist a variety of phenomena (like color contrasts, color shadows, accidental colors, and aspects of color blindness) which posed severe difficulties for the trichromatic theory of Young (1802) and later of Helmholtz (1867/2000). Despite the wealth of observations contained in his Theory of colors few students of vision saw Goethe's theory as other than evidence of the distance that separated art from science.

In a lecture surveying Goethe's scientific researches, Helmholtz attempted to take a sympathetic view by stating that he was primarily a poet, and that he was not disposed to support experimental enquiries into natural phenomena: "Thus, in the theory of colour, Goethe remains faithful to his principle, that Nature must reveal her secrets of her own free will; that she is but the transparent representation of the ideal world" (Helmholtz, 1898, p. 45). Goethe sought to shift the study of color vision away from physics towards phenomenology. Accordingly, he was impressed by the publication, in 1819, of Purkinje's Beitrage zur Kenntniss des Sehens in subiectiver Hinsicht (Contributions to the knowledge of vision in its subjective aspect) and saw him as an advocate of the phenomenological method, although Goethe did display disappointment that his own observations were not cited by Purkinje (see Kruta, 1966). Later Goethe encouraged Purkinje's academic career, although he did not receive the unstinting support from Purkinje that he probably expected.

Purkinje acknowledged Goethe's influence but retained an independent theoretical standpoint. Whereas Goethe had attempted to replace physicalism with phenomenology Purkinje sought to emphasize the physiological dimension. Purkinje's second book on subjective visual phenomena, published in 1825, was dedicated to Goethe. When Purkinje gained access to one of the new large achromatic microscopes, in the early 1830s, he put his observational skills to good use, as is attested by the Purkinje cells in the brain and the Purkinje fibres in the heart.

The laboratory in Breslau where he conducted these microscopical studies has been referred to as "the cradle of histology", and his research was a significant contribution to the development of cell theory and the neuron doctrine. Thus, Purkinje provided not only the conceptual foundations for neuroscience but also the building blocks for its construction. His vision did herald the dawning of neuroscience. Vision The nineteenth century witnessed a revolution in the study of vision - it was displaced from the natural environment to the laboratory. The study of vision was transformed from an observational to an experimental discipline after around 1840 (see Wade, 1998a).

The seeds of the revolution were sown much earlier - in the seventeenth century - with an appreciation of the physieal nature of light and of the anatomical structure of the eye. Kepler (1604, 1611) described the manner in which light is refracted through the eye to form an image on the retina, and Scheiner (1619) provided an accurate representation of the anatomy of the eye. Both Kepler and Scheiner constructed artificial eyes so that the nature of image formation could be examined more systematically (see Park, 1997; Wade, 1998c). The analogy between eye and camera focused interest on the geometrical properties of the retinal image and upon the ways in which two retinal images could be combined. This concern with spatial vision was replaced by the investigation of color phenomena in the eighteenth century, largely as a consequence of Newton's (1704) analysis of the spectrum. Isolating and mixing light of different colors did lend some degree of experimental control to the study of vision, but it generally remained an observational rather than an experimental pursuit.

In the eighteenth century, physics had made advances by isolating variables and then manipulating them, and much the same applied to the study of vision in the nineteenth century. Questions about the nature of vision have been asked since antiquity. For example, why do we perceive the world the way that we do, and how does this come about? In this context the nature of space and time was a central issue in philosophical discussions. Towards the end of the eighteenth century, Kant (1781 ) declared them to be a priori dimensions and thus objects of transcendental aesthetics. This represented a fundamental distinction between his position and that of contemporary empiricist philosophers, like Hume. From Kant's standpoint the perception of space and time were outside the realm of experimental enquiry.

The natural scientists of the early nineteenth century demonstrated, on the other hand, that instruments could be devised which enabled the manipulation of perceived space and time (see Wade & Heller, 1997). The most important of these instruments were the stroboscope, the stereoscope, and the chronoscope; the stroboscope varied space and time together, whereas the other two instruments provided a means for the analysis of space and time separately. Moreover, these instruments proved, contrary to Kant's (1786) assertion, that the study of vision could indeed be scientific. There existed a body of observations concerning phenomena that could be experienced in the natural environment, but there was little in the way of controlling or manipulating the conditions under which they could be seen. This was made possible by the use of the various scopes, and the methods of physics could be applied to the measurements of the senses.

Despite the grand design behind Purkinje's vision his initial experimental work was based on observations of visual phenomena that were made without any elaborate equipment. His studies of vision were conducted before the instrumental revolution took place, and he extended the range of phenomena that can be experienced in the natural environment. The slim volume Beitrage zur Kenntniss des Sehens in subiectiver Hinsicht was his doctoral dissertation which was defended in 1818 and published in 1819. It was reprinted in 1823 with a prefix to the title: Beobachtungen und Versuche zur Physiologie der Sinne (Observations and experiments on the physiology of the senses). The book had a major impact on sensory physiology in Germany, and the phenomena it described continue to be investigated today. Purkinje defined and named a new area of study subjective visual phenomena. This was taken by Goethe to emphasize the subjective dimension of all vision, but Purkinje himself sought to determine the objective correlates of the subjective impressions. In so doing, he set in train the tradition that finds expression in contemporary neuroscience - relating aspects of our experience to their underlying physiological foundations.

Purkinje's book, which is translated in chapter 4, described a range of subjective impressions, some of which were novel and others were steeped in antiquity. Purkinje himself was parsimonious in the references he cited, perhaps because at that early stage of his career he was unaware of the wider literature. Griisser (1984) suggested that Purkinje had access to books on vision by Darwin (1795), Goethe (1810), and Steinbuch (181 l ). Thus, Purkinje was unlikely to have been inhibited in his observations by a burden of received wisdom; this is one of the reasons for the freshness of his descriptions of visual phenomena.

Erasmus Darwin's Zoonomia was translated into German soon after its publication in English. It provided a survey of the senses from a physiological point of view, and related all functions in health and sickness to irritability, sensitivity, volition, and association, and these in turn were discussed in terms of motions of the body parts. Darwin argued by analogy and made many shrewd observations of perception. He commenced his treatise with a statement of intent: "The purport of the following pages is an endeavour to reduce the facts belonging to animal life into classes, orders, genera, and species; and by comparing them with each other, to unravel the theory of diseases" (1794, p. 1 ).

Purkinje was particularly influenced by Darwin's speculations on vertigo. Goethe's Theory of colors presented an attack on the physicalism of Newtonian optics by emphasizing the purity of white light and stressing the subjective dimension of color perception. In line with many Romantic philosophers, Goethe rejected the experimental approach to the study of nature because it was too constrained. In its place he proposed the astute and intuitive observation of natural phenomena, setting in train the method of phenomenology. In the context of color vision, he based his theory on color experience rather than color mixing, and he championed the philosophy of sight rather than the physics of light.

Steinbuch's Beytrag zur Physiologie der Sinne (Contribution to the physiology of the senses) was also influenced by Darwin's Zoonomia, and tried to integrate the psychology of perception with its underlying physiology (see Hatfield, 1990). Steinbuch developed an empiricist and associationist theory of spatial perception based on muscular activity, and he expressed a sentiment linking subjective experience to objective stimulation that is echoed in Purkinje's work. There had been much research on some subjective visual phenomena prior to Purkinje, and this is sketched in chapter 3. The aim of the present section is to examine some of the additional aspects of vision that he examined in his early years.

It is clear that vision was not only his initial but also his abiding scientific interest. Much of the work following his doctoral dissertation was on vision; most particularly a long article on vertigo (Purkinje, 1820) and coverage of a range of visual phenomena in his inaugural lecture at the University of Breslau (Purkinje, 1823b). The inaugural lecture referred to his research on accommodation, peripheral vision, long and short sightedness, and he addressed all these issues in his book of New contributions (Purkinje, 1825a). It is these topics that will be dealt with initially, together with strabismus, the Purkinje shift, and motion aftereffects; a separate section is devoted to his experiments on vertigo.

excerpt from Purkinje's Vision: The Dawn of Neuroscience

Nicholas J. Wade and Josef Brozek in collaboration with Jiri Hoskovec