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The mental process of attention results in a certain duration or memory of the mental condition which is a distinct thing from the primary retinal impression, and leads to the ignoring or mental obliteration of an instantaneous interval separating two phases of the position of moving legs which have strongly "arrested the attention."
Similarly, the projection of an ocular impression due to retinal disturbance into the external field of vision, may rightly be named an hallucination. But the case is not always so clear as this. Thus, for example, when a gustatory sensation is the result of an altered condition of the saliva, it may be said that the error is as much an illusion as an hallucination.
At night the pupil dilates more than in the day-time, and hence vision with the extramacular or peripheral portions of the retina is correspondingly better. It is, therefore, a symptom of serious retinal disease.
Now attention, or vividness, is found to fluctuate periodically, so that in a series of objectively equal stimuli, certain ones, regularly recurring, would be more vividly sensed. This is exemplified in the well-known facts of the fluctuation of the threshold of sensation, of the so-called retinal rivalry, and of the subjective rhythmizing of auditory stimuli, already mentioned.
What is more, in the Salamandra maculata, if the lens be removed and the iris left, the regeneration of the lens takes place at the upper part of the iris; but if this upper part of the iris itself be taken away, the regeneration takes place in the inner or retinal layer of the remaining region.
The retinal vessels stand out slightly in relief, and thus a perfect shadow of the system is cast on the retina. It was this shadow I saw, and the white screen was merely a convenient background for it. I don't know if I make myself clear." "Perfectly clear, perfectly," said the military person. "Indeed, John Medhurst seemed quite loth to part with me.
The appearance of a blending of the stripes of colours on a rotating disc or top, of the formation of a ring of light by swinging round a piece of burning wood, and the illusion of the toy known as the thaumatrope, or wheel of life, all depend on this persistence of retinal impression. Many of the startling effects of sleight of hand are undoubtedly due in part to this principle.
In a certain sense the blind man is right, since vision, having its origin in the stimulation of the retina, by the vibrations of the light, is nothing else, in fact, but a retinal touch. Such is indeed the scientific explanation, for the function of science is just to express all perceptions in terms of touch. Now instinct also is a knowledge at a distance.
Furthermore, the possibilities of damage by hemorrhage from the choroidal or retinal vessels, delayed formation of the anterior chamber and adhesion of the capsule of the lens to the wound, and the injurious effects of even slight trauma subsequent to the operation, including loss of vitreous, are increased by omitting the conjunctival flap.
We have the same intrinsic elements to consider as in natural forms. That is, besides the unity of type and correspondence of parts which custom establishes, there are certain appeals to more fundamental susceptibilities of the human eye and imagination. There is, for instance, the value of abstract form, determined by the pleasantness and harmony of implicated retinal or muscular tensions.
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