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."