In the case of an enlarged heart, Dr. Kelly discovered that a dark spectrum was perceived _synchronous_ with the _systole_, or contraction of _its ventricles_; so that the patient could count his pulse merely by watching the motion of this illusive shade on the white ceiling of his room. The study of these false perceptions, which result from derangement or disease of the eye, are replete with interest. You are aware that the function of a nerve of sensation is so deranged by disease, that in some cases of paralysis cold bodies will appear heated. So, by analogy, is the function of a nerve of _sense_ deranged, if its _fibrillæ_ be disordered. We have _Myopia_, or short sight; _Presbyopia_, or long sight; _Chrupsia_, or coloured vision. We have night-blindness, or dim vision, and day-blindness, or intolerance of light,—as in the albino, or owl. I had, and I have now, a second relative, whose vision is insensible to certain colours; and the chemist, Dalton, we know, could not distinguish blue from pink. In a Glasgow Medical Journal, I read this statement by a patient:—“No colour contrasts to me so forcibly with black as azure blue, and as you know that the shadows of all objects are composed of black, the forms or objects which have acquired more or less of this blue hue, from being distant, become defined and marked by the possession of shadows, which are invisible to me in the high-coloured objects in a foreground, and which are thus left comparatively confined and shapeless masses of colour.” The eye may be curtailed of half its object. Mr. Abernethy and Dr. Wollaston were both often in this dilemma of a sense, so that only one-half of a person or a name, on which they were looking, was visible to them. Mr. Abernethy, in his facetious way, referring to his own name, told us he could see as far as the _ne_, but could not see a bit of the _thy_. This illusion is at once explained by anatomy. The optic nerve, at one point, interlaces some, and crosses other of its fibres: thus _one_ nerve chiefly supplies _one-half of both eyes_. Disease of nerve may thus paralyse _one-half_ of each _retina_: the _other_ half only perceiving half the object or word.