Place a small square of white paper on a large piece of green paper, and cover the whole with a thin transparent piece of tissue paper so that the underlying paper is seen through it. The small square will then appear red, while the surrounding surface seems whitish. If shown to an unprejudiced person, who does not know what is under the tissue paper, he will immediately pronounce the small square to be red, and will hardly notice any colour on the surrounding surface. Under similar circumstances a white square upon a red ground will appear green, a white upon blue, yellow, and vice versa. We might, in this case, be led to imagine that we had merely to deal with the ordinary phenomena of complementary colours, which are produced when the eye, after having been fixed for some time upon a coloured surface, is turned suddenly upon a white one. The white surface will then look red, if we have wearied the retina by gazing for some time at a green light; or vice versa. But, in the case we are speaking of, the phenomenon carinot be explained by fatigue of the retina, because the colour of the larger surface is of so little consequence that it is scarcely recognised, and, therefore, cannot fatigue the eye by its colour. If the tissue-paper is removed, we immediately recognise the small square as white, although the fatigue produced by the coloured surface would be still greater. Phenomena like those just described have been termed Simultaneous Contrasts by Helmholtz. They are explained by our having made an erroneous judgment on what we call white light. We call a body white when it reflects all the colours of the spectrum in the proportions in which they are contained in sunlight M But we are accustomed to slight variations in these proportions, and call such variations white also. This is the case in the experiment under consideration, in which we suppose the coloured surface, which is covered with white paper, to be white. The result is that we do not recognise the true white of the small square as such, but as a white which inclines to the complementary colour. It is very probable that the kind of light which we call 'white' would not remain the same if the proportion of the colours in the light of the sun were to alter; and since we suppose even the sun and its light may not remain the same for ever, it is quite possible that our descendants may have a perfectly different idea of white to that which we now have. It is in these illusions of the organ of vision, and, also, of the other sensory organs, that we are best able to recognise the important part which the Mind plays in the perceptions of the senses, for it is the mind alone which is able to alter the perceptions of the senses, and, thereby, in many cases to create illusions. We see, further, that it is by the activity of the Mind alone that the Sensations of the senses are converted into Perceptions of the senses. For the sensation of the senses, i.e. the excitement of the sensory organ and the passage of this excitement to the brain, does not at all imply the connection of this sensation with a perception of an object or occurrence in the external world. In regard to animals, and especially in pigeons, the observation has been made, that upon the removal of the cerebral hemispheres, in which state they may live for some time, they still possess a sensation of light, which penetrates the eye, and causes a contraction of the pupil; an action which can only be caused by the central organ of the optic nerve in the brain. But a comprehension of the objects seen-i.e. a true perception of the senses-is no longer possible to these animals. They behave like blind animals, run against every obstacle, and no longer possess the power of recognising the objects seen as belonging to the external world. We cannot help conjecturing that the process must be the same in man: that the perception of the external world is essentially an act of the Mind, which has its seat in the cerebrum and is connected with this organ; and, further, that the sensory organ with its nervous connections only affords the brain the material which it converts into a Sensory Perception. Jerund. 1000 ft in Second longitudinally " to if fath chap. 1. sound, 2,3,4, structure Jear 5. Inmate star were. 6; 1, schymichel, theor PART III. THE SENSE OF HEARING. CHAPTER I. General construction of the Organ of Hearing-Sound as a Tone, a Note, THE organ of hearing is not so fully exposed to our The relation of the organization of the ear to that of by an opaque body; in a transparent medium, however, it extends to the infinite distance of the stars, when the ray of light penetrates the eye in an almost straight line to its posterior surface. The extent of our perception of sound, even in our atmosphere, is very limited, but sound can penetrate the thickest walls and travel by the most circuitous paths, so that in the deepest mines where no light can penetrate, the ear gives us certain information. of the existence of an exterior world. Thus, the inner parts of the ear are hidden in a deep cavity in the skull, which is provided with many wonderful labyrinth-like passages. Sound does not penetrate these passages in a straight line, but passes along a very complicated path, composed of tubes, membranes, and the ear-bones. In fig. 53, the essential parts of the organ of hearing are represented, in their natural size, after Helmholtz. We see the Auditory Canal, D, which terminates within B D B CF Fig. 53. with the Tympanic Membrane, c c. This membrane is circular and stretched obliquely, and is followed within by a hollow, filled with air, B, the Tympanic Cavity, |