up our ordinary sensations are themselves compounded of sensations of less intensity and duration, and so on. Thus, there is going on within us a subterranean process of infinite extent, its products alone are known to us, and are only known to us in the mass. As to elements and their elements, consciousness does not attain to them, reasoning concludes that they exist; they are to sensations what secondary molecules and primitive atoms are to bodies; we have but an abstract conception of them, and what represents them to us is not an image, but a notation.

CHAPTER II.

SENSATIONS OF SIGHT, OF SMELL, OF TASTE, OF TOUCH, AND THEIR ELEMENTS.

I. A SIMILAR, though somewhat less complete, reduction may be effected with sensations of sight.* We all know that a ray of white light may be divided with a prism into several rays of different colours. It spreads out into a spectrum, in which the colours form a continuous scale. At the commencement of the scale is red; then come orange and the different yellows, then green, the different blues, indigo, and lastly violet,† and each of these tints passes by intermediate stages into the one preceding it and the one following it. Here are an infinite number of distinct sensations connected by intermediate stages. Let us examine their external conditions. The science of optics shows us that the spectrum is formed by the different rays which make up the white ray being inflected, some more and some less, in passing through the prism; this inflection increases with the shortness and rapidity of the waves; therefore, if we follow, from red to violet, the series of rays which form the spectrum, we find that the shortening and acceleration of the waves go on increasing. Thus, from red to violet, each sensation corresponds to waves quicker and shorter than those of the preceding sensation, slower and longer than those of the succeeding sensation. An increase of speed and diminution of length in the waves are sufficient to determine the variations which our sensation of colour undergoes in passing from red to violet.

* Helmholtz, "Physiologische Optik," part ii.

+ M. Helmholtz distinguishes the following successive colours: red, orange, golden yellow, pure yellow, greenish-yellow, pure green, bluish-green, blue of water, cyanic blue, indigo, violet and ultra-violet,

Having premised this, let us consider the red; as we go down the spectrum, the sensation of red diminishes; it passes from its maximum to its minimum. There is then an elementary sensation, which decreases in proportion as the waves become shorter and more rapid.-But there is more than one such; for if there were only one, we should find that as we passed towards violet, it would simply grow feebler with the shortening and acceleration of the waves, and the entire spectrum would only present degrees of intensity of red, while in fact, we find at what appears to be the minimum of red, a second distinct sensation arising, that of yellow. There are then, at least two elementary sensations of colour.Are there but two? If there were only two, for instance, that of red and that of yellow, the red, having its maximum at the commencement of the spectrum, and the yellow having its maximum at the centre of yellow, the first decreasing with the time and length of the waves, the second decreasing whenever the time and length of the waves are less or greater than the degree of time and length corresponding to the centre of yellow, we should see, on passing down the spectrum below this centre, yellow become indefinitely feebler till the end of the spectrum, without undergoing any other change. This is not so; for at the lower minimum of yellow we find a new distinct sensation appear, that of green.-There are then, at least three elementary sensations, and on studying the composition of the spectrum we find it is sufficient to admit three, one analogous to that of red, another to that of violet, and the last to that of green.

All the three are excited by every ray of the spectrum; but each of the three is differently excited by the same ray. The first is at its maximum at about the central point of red; in proportion as we descend towards the violet and the waves become shorter and more rapid, its intensity diminishes and approaches its minimum.—The second is at its maximum at about the centre of the violet; and as we go back towards the red, and its waves become longer and slower, its intensity diminishes and approaches

its minimum.-The third is at its maximum at about the central point of the green; in proportion as we return towards the red, or descend towards the violet, that is to say, as the waves become either longer and slower, or shorter and more rapid, its intensity diminishes and approaches a minimum. So that, as we pass from red to violet through all the degrees of the spectrum, the three component sensations vary from degree to degree, but each one in a special manner, the first passing insensibly from maximum to minimum, the second from minimum to maximum, the third passing first from a minimum to its maximum, and then from its maximum to a minimum, which explains at the same time the insensible passage by which every compound sensation in the spectrum is connected with the succeeding one, and the diversities of the ten or twelve principal compound sensations.*

We can readily see the object of this disposition of our being. If a simple ray excited in us one sensation of colour only, it would have a maximum, a minimum, and intermediate stages, nothing more; and for want of being able to contrast it with another, we should not observe it ;† we should have no notion of colour; the luminous waves, in increasing or decreasing in speed and length, would only render the sensation more intense or more feeble; objects would differ only in higher or fainter colour; they would resemble the various parts of a drawing in which all the

* Helmholtz, ib. 191. The substance of this explanation is due to Young. He supposes that every nervous fibre of the retina is made up of three elementary fibres, differently excitable by the same ray. As Helmholtz observes, we may suppose that every nervous fibre of the retina possesses three different kinds of activity, excitable by the same ray, and this is very probable.-But we may dispense with all suppositions by admitting, instead of three nervous fibres or three nervous activities, three elementary sensations. In the anatomical or physiological hypothesis, the assumed fact is uncertain; for it is not certain that there are three different fibres in every nerve, or that one fibre has three kinds of action. In the psychological explanation, the admitted fact is positive; for it is certain that the three sensations, red, green, and violet, exist. I have therefore made the necessary changes in the explanation of Helmholtz. "This hypothesis of Young's," he says, gives a complete view and extraordinarily clear and simple explanation of all the phenomena connected with the physiological science of colours."

"Persons affected with achromatopsy can only distinguish degrees of light and dark, they see all objects as they are represented in photography."-Wecker, "Maladies des Yeux," ii. 432.

differences are those of white, grey, and black.-If, on the other hand, every simple ray excited two sensations of colour only, we should still have the notion of colour; we should still distinguish two principal colours, their maxima, minima, intermediates and compounds; but very many of our sensations of colour would be wanting, and their whole arrangement would be reversed. This we observe in studying various cases of illness or congenital infirmity, and the theory reducing our elementary sensations of colour to the three sensations of red, violet, and green, receives here a most striking confirmation from experience.* The sensation of red is wanting in some persons; in others, that of green; after taking santonine, the sensation of violet is lost for some hours. In all these cases, not only is a principal sensation missing, but many others are altered, and both losses and alterations are precisely those which, according to theory, would result from the absence of the elementary sensation. Finally, we obtain a more delicate and definitive verification. According to the theory, the red and violet of the spectrum are, even at the points at which they seem most intense, compound sensations; for, to the elementary sensation which is then at its maximum, are joined two others which are then at a minimum; the first then is mingled and weakened; it is neither absolutely pure nor of the greatest possible strength. It will, then, be purer and stronger if we can remove these causes of impurity and weakness. Now there is a case in which we are able to do this; that is, when we have blunted the sensibility of the eye to the other colours. In this case we ought to see a red or violet more intense than those of the spectrum; and this is what happens. In this instance, which is unique, we

Helmholtz, 294, 848, 293, and Wecker, ibid.-"The ingestion of santonine brings on a particular variety of Daltonism by making the retina insensible to violet rays. Some persons "have no perception of blue; this state always coincides with insensibility of the retina to red rays. Others while distinguishing white, grey, and black from all other colours, do not distinguish other colours from one another. In others, the retina is insensible to violet, while other colours are perceived, if strongly marked and in a bright light."

+ Helmholtz, ib. 369, 370.