Bartholin mentions the case of a hysterical woman who was afflicted with hemiopia horizontally, and saw all natural objects cut in two, the lower halves being invisible. In this instance it was only the left eye that was defective. Another interesting example of optical illusion is the luminous sensation produced internally when the eye, or the neighbouring parts, are struck or stimulated by friction or electricity. These appearances are experienced even by those who have lost their sight. Müller states that a case was submitted to a legal tribunal to decide whether the luminous sensations which are perceptible when we rub our eyes are really light. The matter in dispute was whether a man who was attacked by robbers in the dark, could see and recognise them by means of the light produced in his eyes by a violent blow on the head; but he does not tell us how the question was decided. With regard to internal causes, Humboldt tells us that a man whose eye had been extirpated, was sensible of luminous appearances whenever he was galvanized. Lincke states that a man whose eye had been removed by a surgical operation, saw next day all kinds of luminous phenomena, which tormented him cruelly with the idea that after all his eye had been saved. When he shut the perfect eye, he fancied he saw with the missing eye circles of fire, persons dancing, and similar appearances for several days. These facts are analogous to those told of persons who have had their legs and arms amputated, but who, notwithstanding, apparently feel pain in their lost limbs. ------------------------------------------------------------------------ CHAPTER V. THE APPRECIATION OF COLOUR. MOST people understand each other sufficiently to agree in their ideas about various colours. Thus every one agrees in saying that poppies are red, that the sky is blue, and the leaves green; but if any one were to assert that the sky was red, that the leaves were blue, and poppies green, who could possibly contradict him? This statement may appear a paradox, and an absurdity to many of our readers, but it is really a problem that has engaged the attention of many of our greatest philosophers. Who can prove that what I see as yellow may not appear blue to you, or that what you see red is not green to me? You would possibly explain the doubt by saying that because we both agree in calling a buttercup yellow, that we see the same colour. I call a buttercup yellow, because I have learnt since my childhood to give this name to the particular sensation I experience when I look at one of these flowers; but that is no proof that the sensation I feel is similar to that felt by everybody else, and it is not merely possible, but probable, that our personal sensations of colour are essentially different, although the arbitrary words we use to designate them are the same.