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. Purkinjes 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.
It is much more noticeable when viewed against a field of pure blue light and is caused by white blood cells moving in the capillaries in front of the retina. White cells are larger than red blood cells and can be larger than the diameter of a capillary, so must deform to fit. As a large, deformed white blood cell goes through a capillary, a space opens up in front of it and red blood cells pile up behind. This makes the dots of light appear slightly elongated with dark tails. [4][5]
Haidingers brush is a very subtle bowtie or hourglass shaped pattern that is seen when viewing a field with a component of blue light that is plane or circularly polarized. Its easier to see if the polarisation is rotating with respect to the observers eye, although some observers can see it in the natural polarisation of sky light. If the light is all blue, it will appear as a dark shadow; if the light is full spectrum, it will appear yellow. It is due to the preferential absorption of blue polarized light by pigment molecules in the fovea. [6][7]
Purkinje images are the reflections from the anterior and posterior surfaces of the cornea and the anterior and posterior surfaces of the lens.