Considerations like these add interest to the inquiry—What variety and extent of information do we, as intellectual, immortal beings, gain through our senses? A little thought will convince us that, without the reflective agency of our minds on the impressions received by the senses, and the power of conceiving abstract ideas, the simple impressions of colour, solidity, weight, sound, flavour, or odour, would be but meagre sources of knowledge. As it is, they are the broad inlets of knowledge. Let us take them in succession, commencing with a new chapter.
[CHAPTER III.]
THE SENSES, AS THE INLETS TO KNOWLEDGE—SIGHT AND HEARING.
1. Sight.—The information acquired by the mind through the sense of sight is more varied than that communicated through any other sense, singly taken; but it is less accurate. The sense of sight requires education and assistance, and both are given through the sense of touch. We have already said, that light radiates from bodies, and forms a picture on the retina, (a reversed picture,) conveying to the mind certain qualities of matter, and, first—colour. By variations in the shades of colour, and by contrasts of colour, our eye, aided by experience from the sense of touch, determines the form of objects; and the boundaries of these forms we term outlines. All that we perceive is the limits of differently coloured spaces. Where, however, colours blend gradually into each other, the limits of each space, and, consequently, its outline, are indefinite; we do not see matter, but a quality of matter. There are circumstances under which the eye is deceived respecting colour, two or more different colours producing a similar effect on the retina, and being undistinguishable from each other. Moreover, when the retina is fatigued by a prolonged impression from one colour, its sensibility to colour is diminished, and we cease to judge accurately; in these instances, the retina receives those only which are termed accidental, namely, which do not enter into the colour steadily gazed at previously. For example, "if we look steadily at a white spot, and afterwards turn the eye towards white bodies, a dark spot will be perceived by the eye. If we look at a red spot on a white ground, and then direct the eye to another part of the white ground, a green spot approaching to blue will be perceived. In the first case, the retina was fatigued by the white colour, and could not be excited by any other colour having the rays which constituted it in its composition. The accidental colour, therefore, was black. In like manner, after looking at the red spot, the retina was insensible to the impressions of a compound colour, having red rays in its composition. Hence, the accidental colour consisted of the other rays of the prismatic spectrum, forming a colour destitute of red."
The eye is subject to deception, or illusion. Sometimes we mistake the shadow for the substance. The rays of light, passing through media of different densities, are refracted at different angles; hence the direction or nearness of different bodies becomes apparently altered, and experience alone corrects the error. On putting a pole or stick obliquely into clear water, the submerged portion seems to be bent at the surface of the water, and to rise higher in the water than it actually is. The bottom of a deep, clear river, or pond, seems to be nearer to the surface than it is in reality; in other instances, the water deceives by an apparent shallowness, the truth of which the inexperienced bather should always test before venturing in. If a gold or silver coin be put at the bottom of a basin, and the basin be then filled with water, and the eye be obliquely directed, so as not to see the coin, hidden by the rim of the basin, it will become apparent when the basin is filled. In spearing fish, allowance is made by the experienced for the difference of refraction of the rays of light in a rare and more dense medium. It is probable that birds which prey upon fish, by darting at them while at a certain distance under the water, as, for example, the fish-hawk, or osprey, and the gannet, are taught by instinct to correct an apparent error of sense, or rather, an illusion, which man corrects by practice and experience.
The eye is deluded by the mirage. The mirage (a French word, adopted into our language) is "the name given to a phenomenon of unusual attraction, for which we have no specific appellative, unless it be the sea-term looming. As a general definition, we may say, the mirage is an optical illusion, occasioned by the refraction of light through contiguous masses of air of different density, such refraction not unfrequently producing the same sensible effect as direct reflection.
"The illusions of the mirage differ according to circumstances, but they may be all arranged under one or other of the three following classes: vertical reflection, horizontal or lateral reflection, and suspension."
In hot, flat regions, as certain portions of Arabia, Egypt, Persia, the western deserts of India, etc., the vertical mirage is extremely common; it presents the illusive appearance of a wide, clear lake, or sheet of water, in which trees, buildings, and other objects, seem to be reflected, and in a reversed position. On approaching this deceptive lake, it keeps receding as we advance, the reflected images vanish, to be succeeded by others, as they come in rotation into sight. And thus the wayworn and thirsty traveller across the desert is first cheated into hope, and at length mocked into despair. During the French campaign in Egypt, under Napoleon, the soldiers, who suffered extremely from the torments of thirst, were cruelly tantalized by the deceptive mirage. M. Monge, one of the savans who accompanied the army, thus comments upon this phenomenon: "The soil of Lower Egypt," he observes, "is a vast plain, perfectly horizontal, its uniformity being interrupted by a few eminences, on which, in order to secure them from the inundations of the Nile, the villages are built. In the morning and the evening the aspect of the country presents nothing remarkable, all objects appearing in their natural positions, and at their proper distances; but after the soil has become heated by the rays of the sun, the prospect seems bounded by a general inundation. The villages at a little distance appear as islands in the midst of an expansive lake, and the image of each village is seen invertedly reflected, as if the water were real. As we advance, the mimic water retires, still reflecting image after image;" so that in this illusion the classic fable of Tantalus is represented; might not the fable have had its origin in the phenomenon of the mirage?
It is not, however, exclusively on sandy plains or in very hot climates, that this mirage occurs. It has been observed by Biot over the sandy beach of Dunkirk, and is not unfrequent along the coast of Calvados. A strange and beautiful effect of the mirage was seen by captain Maundy, at the Shallout Pass in India, and is thus described: "A deep precipitous valley, at the bottom of which I had seen one or two miserable villages in the morning, bore in the evening a complete resemblance to a beautiful lake. The vapour which played the part of water, ascended nearly half-way up the sides of the vale, and on its bright surface the trees and rocks were distinctly reflected."