The Massachusetts Spy, And Worcester County Advertiser

THE END

Intensity of light is necessary to distinct vision, but the degree varies with the previous state of the organ. A person passing from the bright day into a shaded room, for a time may fancy himself in total darkness; and to persons sitting in the room and becoming accustomed to the less light so as to see well with it, he will appear to be almost blind. The dawn of morning after the darkness of night appears much brighter than an equal degree of light in the evening. When, as the night falls, our lamps or candles are first introduced, the glare is often for a time offensive: and the same feeling is still stronger on opening, in the morning, bed-room window-shutters or close drawn curtains. After the repose of night. the sensibility of the eye is such that the globules of blood in the capillary vessels of the retina produce the impression on it of little globes of light crossing among each other as the tortuous vessels do.

To a prisoner after long confinement in a dark dungeon, the light of the sun is almost insupportable. And a dungeon, which to common eyes is utterly dark, still to its long held inmate has ceased to be so. There are various instances in the records of the barbarous ages, of prisoners confined for years in utter darkness. who at last could see and make companions of the mice which frequented their cells. The darkness of a total eclipse after bright sunshine appears much more deep than that of midnight, because of the contrast. The long polar night of months ceases to appear very dark to the polar inhabitants.

If an eye be directed for a time to a black wafer laid on a sheet of white paper, and afterwards to another part of the sheet, a portion of the size of the wafer will appear brilliantly illuminated; for the ordinary degree of light from it appears intense to the part of the eye lately receiving almost none. An eye directed long and intensely upon any minute object; as when a sailor watches a speck in the distant horizon, supposed to be a ship, or when a hunter on the brown heath keeps his eye fixed on some game nearly of the color of the heath, or when an astronomer gazes long at a little star—has the sensibility of its centre at last exhausted, and ceases to perceive the object; but on directing the axis of the eye a little to one side of the object, so that an image may be formed only near the centre, the object may be again perceived, and the centre in the mean time enjoying repose, will recover its power.

But the most extraordinary fact connected with the sensibility of the retina is, that if part of it be strongly exercised by looking for a time at an object of any bright color, on then turning the eye away or altogether shutting it, an impression or spectrum will remain of the same form as the object lately contemplated, but of a perfectly different color. Thus, if an eye be directed for a time to a red wafer laid on white paper, and be then shut or turned to another part of the paper, a beautiful bright green wafer will be seen, and vice versa, a green wafer will produce a red spectrum, an orange wafer will similarly produce a blue spectrum, a yellow one a violet spectrum, &c. ; and a cluster of wafers will produce a similar cluster of opposite colors. If the hand be then held over the eyelids to darken the eyes and prevent entirely the approach of light, the spectrum of the bright parts will be luminous, surrounded by a dark ground, and when the hand is again removed the contrary will be true.

Again, if the eye be in a degree fatigued by looking at the setting sun, or even at a window with a bright sky beyond it, or at any very bright object, on then shutting it, the lately contemplated forms will be perceived, first of one vivid color, and then of another, until perhaps all the primary colors have passed in view. These extraordinary facts prove that the sensation of light and color, although excitable by light, is also producible without it.

This truth gave occasion to Darwin's ingenious theory, that the sensation of any particular color, of red for instance, is dependent upon a certain state of contraction of the minute fibres of the retina, as the sensation of a particular tone depends on a certain frequency of vibration of some part of the ear,—and that the fibres, when fatigued in that condition, seek relief when at liberty, by throwing themselves into an opposite state,—as a man whose back is fatigued by bending forward, relieves himself not by merely standing erect, but by bending the spine backwards—which new condition, whether produced by light or any other cause, gives the sensation of green. He applied his explanation similarly to all other cases of color. It is remarkable that the colors which thus appear opposite to each other in kind are those which when the solar spectrum produced by a prism, is painted round a wheel or circle, are opposite to each other in place.

Arnott's Physics.