I am compelled to prolong my glance to call them up precisely and completely, to imagine the muscular sensations of my three steps, the muscular and tactile sensations of my hand, passed along the edge of the table. I only get at this by dwelling on it, by silently inquiring of myself what it is I mean by this distance and this form. Even when dwelling on it, I commence by imagining the first step I should take, the sensation which my hand would receive from the first corner; these two images serve as type for the rest. In fact, my operation is the same as when in a written sentence I read the word tree; if I read it rapidly, I simply understand it; it does not call up express images in my mind; it is necessary to weigh it, to reflect, in order to cause the image of a beech, an apple, or other tree to appear; even then, it will be vague and mutilated; at the most, I shall get a view of some lineaments of a coloured form, the obliterated sketch of a green dome or pyramid; it will only be by dwelling strongly on it for some time, that I shall cause images of trees to spring up sufficiently clear and numerous to be equivalent to the generic word which sums up and denotes them all.—Thus our optical sensations are, like our words, signs. Every retinal and muscular sensation has, like every word, its group of associated images; it represents this group; it replaces and denotes it; in other words, it is always associated with it, and never otherwise associated, so that in use and practice it is equivalent to it. In fact, when the sensation arises, the group is at hand, ready to revive. Give it a little time, and it will partially revive. Give it enough time, and it will wholly revive. It forms part of the train of the sensation; but, as the operations are rapid, it most usually remains in the background; the sensation alone occupies the stage. As the sensation is there for an instant only, and the train requires time to pass in procession, the train remains behind the scenes.-We know something of this world behind the scenes.* The reader had a view of it when

* See part i. book iv. chap. i. pp. 178-80.

we pointed out the silent persistence of images, their latent existence, their rudimentary state, the obliteration they undergo, and the life they preserve, often for whole years, until the indistinct vibration, which was perpetuated in some only of the cells of the hemispheres, receives from some unforeseen circumstance an universal ascendancy, and becomes suddenly propagated through the majority of the chords of the cerebral instrument.

The better to comprehend their obliteration and the part they still play, though in this latent state, let us consider greater distances, and generally, the process by which distances are estimated.—In a geographical map we look at the myriametre traced at the foot, and applying compasses to this myriametre, we pass about the map, measuring in this manner if Paris is farther from Bourges than from Tours or Dunkirk.-At the outset of the operation, we estimated the myriametre in muscular sensations; and found it equivalent to some walk we have been accustomed to take, to 12,000 paces, to two hours' walking. But, soon after, we forgot the muscular signification we attached to this expansion of the compasses; we left it behind us, in reserve; all we kept in mind was degree of expansion and its multiples; we directly compared a series of expansions to a series of expansions, a series of greater ones to a series of less. We follow the same process in all our estimations of quantities, and the spontaneous operations of our eye do but precede the artificial operations of our instruments. In the first stages of our observation, as at the limit of our science, we prove a constant relation between two quantities, just now between our more or less numerous steps and the greater or less expansions of the compasses, at present between the more or less long and repeated muscular sensations of our limbs, and the muscular sensations which we receive from the greater or less convergence of our eyes, the greater or less flattening of the crystalline lens, the greater or less contraction of some one of the muscles which move the eyes, the greater or less movement of our body and head in some particular

direction. The second quantity increases or decreases with the first, according to a fixed law. This being settled, we take a standard of the second, just now a certain expansion of the compasses, for instance the expansion which measures the myriametre, now a certain muscular sensation of our optical apparatus, for instance, the muscular sensation which the eye must experience in order to have the retinal sensation of an object situated at thirty centimetres distance. At this moment too, the standard and its signification, that is to say, the expansion of the compasses and the recollection of our walk, that is to say again, the muscular sensation of the eye and the image of the muscular sensation of the arm carried thirty centimetres forward, are together in our mind. But a moment afterwards, the standard alone remains; the image or the recollection to which it is equivalent becomes thin, and fades away; we simply observe that a certain expansion is greater than another, that a certain muscular sensation of the eye is stronger and more prolonged than another; we no longer perceive the signified quantities but only the significant ones. This is enough; for, by means of the indicated association, the signified quantities remain within call, and their proximity is as good as their presence. At any

moment we can recall them, can observe that a certain expansion of the compasses, that is to say, one of three times the extent of the first, would require of us three times as many steps, or six hours' walking, that a smaller muscular sensation of the eye would require a double extension of our arm.-We know how a map serves us on a walking tour; by applying to it compasses, we foresee the length of our walks, and the amount of muscular effort we shall be compelled to expend. Our visual atlas is of the same use; by translating certain of its indications into the corresponding indications of the tactile and muscular atlas, we foresee the distance, the magnitude and duration of the muscular effort by which our limbs will reach some particular object.

VI. We see now how it is that a visual sensation, so short

as to appear instantaneous, can give us the idea of very diversified and very great extension. This arises from its being equivalent to the very diversified and very long tactile and muscular sensations by which we should perceive this extension. It sums them up, and so becomes their substitute, and signifies them while replacing them.

But, even were we incapable of having this sensation, we should still contrive to represent to ourselves in combination, and as simultaneous, a great number of the parts of space. I have questioned many blind persons as to this ;* space.—I their answer is unanimous, wholly precise and decided. No doubt, the perception of a new object requires more time in their cases than in ours, since they are compelled to explore it in detail by touch. But, having done so, they think of the object, whatever it be, a sphere, a circle, or even a considerable space, for instance, a street, all at once, and represent it to themselves in mass. "All there is wanting to us," they say, "is what you call the idea of colour; the object is to us just what an unshaded drawing or photograph is to you, speaking more precisely, a combination of lines. We conceive a whole group of diverging or intercrossing lines simultaneously, and that is to us figure." Above all, they expressly deny, that in order to imagine a line or a surface, they require to represent to themselves the successive sensations of their hand passed in some particular direction. "That would be too long, and we have no need whatever to think of our hand; it is but an instrument of perception of which, after perception, we cease to think."

In fact, if, at the origin of the idea of distance, we find a longer or shorter series of muscular sensations of the arm or leg, it is at the origin only. It matters little whether these sensations appertain to one limb or the other, whether they are muscular or not; this is but a detail and an accessory; it is obliterated, we cease to attend to it. We leave aside, as the blind say, all the circumstances and

*At the Institut des Jeunes-Aveugles at Paris, thanks to the kindness of the Professors and Director of the establishment.

more.

intrinsic qualities of our sensations; we preserve only the essential part, and the essential part here, is that they form a series interposed between the two points whose distance we are estimating. Taken thus abstractedly, these sensations become, as it were, uncoloured and neutral; they are any sensations whatever; we consider them not as to their quality but as to their quantity; what we observe in them is the greater or less duration of their series; nothing Henceforward, we are able to imagine them with great promptitude, and to compare series to series. Such is the process employed by a person born blind; he may, like Saunderson, become a geometrician, may conceive longer or shorter series, diverging according to such and such an angle; these are lines, and, by a combination of such lines, he conceives geometrical bodies. For our own parts, we avail ourselves of this process when we define lines by the motion of a point, the surface by the motion of a line, the solid by the motion of a surface, and when we estimate a line, a surface, or a solid by the greater or less prolongation of the muscular operation, which engenders its perception. Now we can imagine these movements with extreme rapidity; we may, then, thus, by these means alone conceive many lines, therefore a surface, and even an entire solid, almost in an instant.

But, fortunately, we have a second aid, the visual atlas, which is added in our cases to the tactile and muscular atlas. Thanks to it, we have at our disposal new series which may be compared together, and whose elements succeed in us with prodigious velocity. These are the little muscular sensations of the eye, which are extremely short, and able, therefore, to denote, in an imperceptible portion of time, very great distances, and positions as numerous as various. They take the place of the muscular and tactile images corresponding to them, and, as they pass in a moment, it seems as if the much longer series of tactile and muscular sensations has also taken effect in a moment. Their muscular and tactile signification springs up with them, and we imagine that we perceive at once a number of distant