It is an imperative belief of the natural scientist that each and every modification of a subjective state in the sphere of the senses corresponds to an objective state.
— Purkinje (1819, 1823a, p. 92)
Dawn heralds a transformation of vision. The brightnesses of colors change differentially: Blue objects that appeared brighter than red ones before sunrise reverse thereafter. This phenomenon must have been seen throughout the history of humankind, but Purkinje observed it and committed it to print in 1825. It is now called the Purkinje shift, and it can be related to the different spectral sensitivities of rod and cone receptors in the retina. Purkinje created a shift in more than our appreciation of colors: He shifted the way in which we think about vision itself and of the links with our underlying biology. His vision was that all subjective experiences have objective correlates. This was the dawn of neuroscience.
In the quest to achieve his vision, Jan Evangelista Purkinje or Purkyně (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