This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Effects Of Pressure On Coloured Spectra afford an interesting illustration of the fact that mechanical irritation of the retina produces a sense of light, and not of pain. I gaze on the flame of the lamp beside me, and on shutting my eyes I see a spectrum, which, instead of being of the complementary colour, is bright yellow, with a red margin, and floating on a dark green halo. I press my Angers against my closed eyes, and obtain the complementary colour, namely, a violet spectrum with a green margin on a yellow halo. On removing the pressure the original colours return; and this can be repeated several times. The appearances are even more complex, if the experiment be made with sunlight.
Phenomena of light uncomplicated with colour can likewise be obtained by mechanical irritation. An accidental blow on the eye produces an appearance of sparks of fire; and by gentle pressure the effects called phosphènes are obtained. A phosphène is a luminous image produced by shutting the eyes, and touching one of them lightly but firmly on the outer, inner, upper, or lower border—in short, on any part where the retina extends. A luminous crescent, or complete circle, flashes into sight at the point diametrically opposite the pressure. This is called the larger phosphène, and is caused by irritation of the retina at the point touched, referred by the mind, like all retinal impressions, to the position vertically opposite. Besides this, a smaller phosphène may be obtained, visible at the part touched, which is caused by the contents of the eyeball being pressed against the opposite point. The smaller phosphène is a blush of light of variable intensity, extending over a space, larger or smaller, according to the size of the object with which pressure is made; the larger phosphène is always brilliant, evanescent, and confined to a ring. phosphènes are much more easily produced at one time than another; and after reading to a late hour, the mere closure of the eyelids in a dark room may cause a bright circle of light to flash before each eye.
By means of pressure, patterns produced by a number of internal structures of the eyeball can be brought into view. The branches of the retinal artery may thus be seen as dark or luminous lines; also another pattern, which appears to be the network of the choroidal capillaries; and sometimes patches of small points closely set together, which have exactly the appearance of the extremities of the rods and cones of the retina itself. These experiments may be carried to the extent of giving pain, and are certainly bad for the eyes.
The retinal vessels, however, can be seen in a less unpleasant way, by holding a light a few inches from the side of the eye in a dark room, and gazing forwards into the darkness while the light is gently moved. In a little while the field of vision becomes yellowish, and dark lines are seen ramifying in it in the position of the branches of the retinal artery. These are what are termed figures of Purkinje, and are occasioned by the vessels intervening between the light and the rods and cones behind them. The blood corpuscles, as they traverse the anterior layers of the retina, can also be seen as luminous spots, when one gazes intently into a clear sky.