and forth in the field of the open eye. Then one should see the sixth Purkinje as a dimmer image moving in the opposite direction. The Purkinje tree is an image of the retinal blood vessels in one's own eye, first described by Purkyně in 1823. It can be seen by shining the beam of a small bright light through the pupil from the periphery of a subject's vision. This results in an image of the light being focused on the periphery of the retina. Light from this spot then casts shadows of the blood vessels (which lie on top of the retina) onto unadapted portions of the retina. Normally the image of the retinal blood vessels is invisible because of adaptation. Unless the light moves, the image disappears within a second or so. If the light is moved at about 1 Hz, adaptation is defeated, and a clear image can be seen indefinitely. The vascular figure is often seen by patients during an ophthalmic examination when the examiner is using an ophthalmoscope. Another way in which the shadows of blood vessels may be seen is by holding a bright light against the eyelid at the corner of the eye. The light penetrates the eye and casts a shadow on the blood vessels as described previously. The light must be jiggled to defeat adaptation. Viewing in both cases is improved in a dark room while looking at a featureless background. This topic is discussed in more detail by Helmholtz. Purkinje's blue arcs are associated with the activity of the nerves sending signals from where a spot of light is focussed on the retina near the fovea to the optic disk. To see it, one needs to look at the right edge of a small red light in a dark room with the right eye (left eye closed) after dark-adapting for about 30 seconds; one should see two faint blue arcs starting at the light and heading towards the blind spot. When one looks at the left edge, one will see a faint blue spike going from the light to the right. A phosphene is the perception of light without light actually entering the eye, for instance caused by pressure applied to the closed eyes. A phenomenon that could be entoptical if the eyelashes are considered to be part of the eye is seeing light diffracted through the eyelashes. The phenomenon