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I must here apprize the reader, that it is very difficult for different people to give the same names to various shades of colours; whence, in the following pages, something must be allowed, if on repeating the experiments the colours here mentioned should not accurately correspond with his own names of them. I. Activity of the Retina in Vision.
Whereas a due exercise of the faculties of the mind strengthens and improves those faculties, whether of imagination or recollection; as the exercise of our limbs in dancing or fencing increases the strength and agility of the muscles thus employed. If the muscles of any limb are inflamed, they do not move without pain; so when the retina is inflamed, its motions also are painful.
At the same time, the little sensitive spot at the bottom of the eye-pit spread out into the shape of the bottom of a cup, called the retina; and then the hollow of that cup between the retina and the cornea filled up with a clear, soft, animal jelly called the vitreous humor, and we have the eye as it is in our heads to-day.
Gibbie, who did not believe he had been seen, stood laughing on the floor, when suddenly he heard the key entering the lock. He bolted through the cat-hole but again just one moment too late, leaving behind him on Fergus's retina the light from the soles of two bare feet.
Thus after looking at any luminous object of a small size, as at the setting sun, for a short time, so as not much to fatigue the eye, this part of the retina becomes less sensible to smaller quantities of light; hence when the eyes are turned on other less luminous parts of the sky, a dark spot is seen resembling the shape of the sun, or other luminous object which we last behold. See Sect.
The point which is of importance is not the length of time during which the stimulation of the retina caused by an image endures becoming weaker and weaker as fractions of a second pass but it is this: How long will a stimulus last in undiminished brightness? If it has had time to decline in intensity, the appearance of flickering results. That is what the cinematographer has to avoid.
There are a few with whom it is always morning, and others who remember something of the radiance of the young day even in the heart of midnight. These disprove the postulates of sameness and satiety, these are not smitten by the seen fact as are you of the microscopic retina, these "see life steadily and see it whole." We need not fear the label of an idea.
The internal surface of this membrane secretes a dark substance resembling black paint, upon which the retina is spread out, and which is of great importance in the function of vision, as it seems to absorb the rays of light immediately after they have struck upon the sensible surface of the retina.
Indeed, what mortal is there of us, who would find his satisfaction enhanced by an opportunity of comparing the picture he presents to himself of his own doings, with the picture they make on the mental retina of his neighbours? We are poor plants buoyed up by the air-vessels of our own conceit: alas for us, if we get a few pinches that empty us of that windy self-subsistence!
And if it be supposed, that the action of the retina in producing the perception of any colour consists in so disposing its own fibres or surface, as to reflect those coloured rays only, and transmit the others like soap-bubbles; then that part of the retina, which gives us the perception of snow, must at that time be white; and that which gives us the perception of grass, must be green.
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