not sufficiently sensitive to be influenced by it; it is, therefore, just probable that there are animals whose eyes are so much more sensitive than ours, that they can see rays that exist far beyond those seen by us. Now, as difference of colour is produced by difference in the rate of vibration, it follows that those whose eyes are sensitive enough to perceive the extreme violet rays, see tints of violet that are inappreciable by others.
The power of sulphate of quinine in reducing the luminous vibrations is easily seen by passing a tube filled with the solution successively through each of the colours of the spectrum formed by a quartz prism; the ordinary colours will pass through the liquid as if it were simply water, but on arriving near the violet extremity a gleam of pale blue light will shoot across the tube, and continue to increase. As it is moved onwards the light will gradually die away, until a point is reached nearly equal in length to the whole of the visible spectrum, when it will disappear altogether. It is somewhat singular that no substance has yet been found that will increase the refrangibility of the dark rays beyond the red end of the spectrum. There are many artificial flames which produce this dark light (if we may use such a paradoxical expression) in greater quantity than the sun, whose light is no doubt greatly deteriorated in this respect during its passage through the atmosphere. The substance of which the prism is made also greatly influences the length of the invisible portion of the spectrum. By using a quartz prism and lenses of the same material Professor Stokes, found that the spectrum of the electric light could be traced for a distance equal to six times that of the visible portion.
The action of certain substances in rendering the invisible rays of light perceptible may be easily shown by any one possessing a horse-chestnut tree. A weak decoction of the inner portion of the bark having been
