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Rh intercepted and reflected at the same angle as that of incidence. When these same rays strike the eye of man, the illuminated body becomes visible.

2nd. All light contains three principal colours—blue, yellow, and red. Now, if the surface of the body on which the rays fall are of such nature as to reflect these three colours in equal mixture, the body reflected on will appear white. If the nature of the object has quality which decomposes these three rays, sometimes it absorbs two, and then the object appears of the colour of the third; but if it absorb one only, then it will represent a colour partaking of the two remaining colours; and from the respective degrees in which one or other of these primitive colours predominate, arise the variations of colours. Lastly, if all the three colours are absorbed by the object, it appears black. The rainbow shows an arrangement of the chief colours, and may be recollected by the word vibgyor—i.e. violet, indigo, blue, green, yellow, orange, and red—which may be regarded as the primitive and mixed colours.

3rd. When light falls on transparent bodies, such as water or glass, at right angles on its surface, it proceeds straight through, and is not intercepted by the substance; but when it falls obliquely, it suffers refraction, and the greater the density the greater will be the refractive power. The old experiment, of putting a stick in water or oil, will demonstrate this rule. The first two rules render visible the infinite variety of objects and colours in the external world; and by the third, as applied in the mechanism of the eye, man is enabled to see these objects and colours. Although we can say nothing new, yet we should endeavour more fully to explain the process of seeing, and in familiar terms. The first rule shows the rays of light must reach the eye in a diverging direction, when it is turned towards the objects, and these rays have the form of a cone, whose