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 Appearances Of Solidity And Hollowness depend partly on the apparent diminution of receding objects, partly on the way in which the light falls, and partly on the pictures presented to the two eyes being different, and bringing into view a larger amount of surface than can be seen from one point. The last of these three causes is supplemented materially by the other two; otherwise the appearance of solidity would be lost on shutting one eye. It is, however, a most important element, as is shown by the effects of the stereoscope, when geometrical figures are looked at.
I have already pointed out how it is that only one figure is seen when two pictures are looked at through the stereoscope. But stereoscopic effect depends on the circumstance that no solid or hollow body presents exactly the same view to both eyes. The artist provides on the stereoscopic slide two views of one object, such as would be presented in nature to the two eyes; and the eyes are directed by the construction of the instrument, so that nearly similar parts of the pictures fall on identical points of the retina. In looking at a solid object, the portions of its sides brought into view are seen to a greater extent by the eye on the same side than by the other; but in a hollow object, they produce the broader image in the eye on the opposite side; and thus it happens that if the stereoscopic views of a solid body be clipped separate, and each be placed in the instrument in the position which was intended for the other figure, it is made to appear as a hollow. This effect can be obtained in perfection with geometrical figures without shading; but is aided by the reversal of the shading when an irregular figure is looked at. By means of another instrument, the pseudoscope, the rays coming from actual objects are directed in such a manner that the image which should be presented to one eye is made to fall on the other, and by this means raised objects seem as if hollow, and vice versa. But the important part which experience plays in giving the idea of solidity and hollow-ness, is shown by the circumstance that neither with stereoscope nor pseudoscope can the reversal of appearance, or conversion of relief as it is termed, be obtained when the objects looked at are of a complex description, and so appeal to our associations that they cannot be conceived otherwise than as they really are:
183. It has already been pointed out that impressions on the retina have a certain tendency to diffusion; they have likewise a tendency to endure after removal of the stimulus, particularly if this have been applied with much intensity, or for a length of time. After gazing at the sun, or any bright light, a spectrum or coloured figure remains before the eye for some time; and any object, whether brilliant or not, if moved rapidly, can be shown to leave impressions on the retina which endure after cessation of the stimulus. When a wheel of a carriage is in motion, the spokes become indistinguishable one from another, and a dull tinge of their colour, brightest near the nave, is diffused round about; because the spokes each affect the retina in every part of their revolution, the impression made by one at any part being immediately succeeded by another, and the spokes are placed most closely together near the centre. This imperfection of vision would be much greater, were it not that the eye has a tendency to follow any object on which the gaze is fixed. The eye follows the wheel of a carriage, and the image continuing to be made on one part of the retina is correctly appreciated as circular; but it fails to follow the spokes, and therefore these affect successive parts of the retina with great rapidity. The duration of retinal impressions is the principle on which a number of optical toys depend. The simplest of them are cards with pictures on each side, and twirled round by a couple of strings, one at each end, so as to bring the pictures together. Thus, a bird on one side and a cage on the other gives the appearance of a bird within a cage; and a man riding a horse may be brought into view, the man being painted on one side of the card and the horse on the other. The thaumotrope and the anorthoscope are instances of much more complex toys dependent on the same principle (see Glossary).
The coloured spectra seen after gazing on bright objects are connected with something more than the mere duration of impressions, namely, the power of appreciating different colours. The appreciation of colour is not understood; we do not know by what mechanism the rods and cones are thrown into different conditions by lights of equal intensity but different wave lengths, so that both colour and intensity are appreciated by one set of structures. But it is known that the appreciation of colour is a separate power from the appreciation of light and shade, for there are various kinds of colour-blindness which occur uncomplicated with other defect of sight.