images and the comparison of their positions in the two fields. With respect to the discrimination of the images of the fields which serves as a basis for the perception of solidity, the eyes' judgment is found to be most exact in relation to objects lying in the horopter, and becomes less and less exact as the distance from this increases. Thus the perception of relief is particularly exact in the plane of the ground. This may be seen by comparing the exact stereoscopic impression given by this plane under ordinary circumstances with the impression made by looking at the horizon with head bent sidewards or, still better, with head bent down so that objects are looked at between the legs.* In these cases, as Helmholtz points out, we see "the farther portions of the ground no longer as horizontal but as a wall painted on the surface of the sky." The element of solidity and relief being much better appreciated in the case of objects lying in the horopter, we are accustomed instinctively to bring objects which we have to observe carefully, as far as possible, into the horopter.

Let us now pass to the second mode of comparing the two fields, viz., that which subserves the perception of the apparent distribution of objects in the common field of vision, and the discrimination of double images. This is found to be exact only in the middle of the field, being liable to be very inexact in the peripheral regions. The conscious separation of double images is rendered impossible by a number of psychical conditions, foremost among which is a pre-existing conception of the unity of the object which projects the images. Certain precautions have to be taken in most cases in order to recognise double images at all, and even then the comparative estimation of spacial magnitudes by this means is much less exact than that of similar magnitudes in a single field.

The reader must be referred to Helmholtz's work itself for a full account of the circumstances which affect and limit this power of recognising double images, or in other words the conditions which determine whether the two images are fused in a single perception or recognised as double. The experiments in combining stereoscopically pairs of drawings, which are here described, are exceedingly interesting, and can easily be carried out by every reader for himself even without the aid of a stereo

It is well to mount a stone or hillock so that the altitude of the head above the plane be not materially altered.

† Helmholtz refers the increased brilliance of the colours of a landscape when looked at in this way to the change effected in the perception of relief. So long as this is not disturbed the modifications of the colours of objects by the atmosphere are looked at as the customary attributes of distance, and not attended to in themselves.

scopic apparatus. Here it must suffice to name one or two of the most interesting facts.

The main ground for the coalescence of images which do not fall on corresponding points is their degree of resemblance to the two perspective images projected on the retinas by one and the same object. The greater this is, the more difficult it is to perceive them as two. Double images may often be recognised by means of a strenuous volition aided by a vivid representation of their plurality. Again the recognition of double images as such may be facilitated by the addition of most insignificant incongruities to the two pictures or designs which are to be combined. Once more it is shown by a series of experiments undertaken by Volkmann that images only coalesce when their vertical distance is small. On the other hand the limit of horizontal distance is much greater. It is hardly necessary to add that practice greatly improves the power of distinguishing double images.

Among the most striking facts brought to light in these investigations is that mentioned by Wheatstone, viz., that just as the images of disparate or non-corresponding points may coalesce, so under certain circumstances the images of corresponding points may be projected in different directions and so seen double. This fact has been disputed, but Helmholtz shows that it is a necessary consequence of the coalescence of images of disparate points.

It may be asked whether movement of the eyes is essential to the coalescence of images and to stereoscopic perception. Brücke broaches the theory that all perceptions of depth are gained by movement, and that double images are only got rid of by successively fixating the single points and so seeing them simply. Yet it has been found by Dove that this combination takes place in many instances instantaneously when the pictures are illumined by an electric spark.* At the same time Helmholtz holds that with the wandering of the eyes over the object the intuition of depth or solidity becomes decidedly more exact and vivid than with the fixation of a point. This he explains by saying that we only perceive difference of depth or distance very nicely when the points happen to fall in the particular horopter of the moment.

In the foregoing investigations the double images resembled the perspective images which are usually received from one and the same object, and in consequence were easily combined as signs of this object. When, however, they are altogether dif

That ocular movement is not essential is proved also, as Helmholtz tells us, by the fact that after-images or spectra may be combined stereoscopically.

ferent, having no such perspective relation to one another, this combination becomes impossible. When, for example, the two fields are filled with quite dissimilar forms, there is no question of combining the impressions in a single perception. Here it is simply a question of seeing the two fields, or one rather than the other. This subject has been studied under the title of "Rivalry of the fields of vision". It concerns us here only so far as it helps to throw light on the nature of the correspondence of the two retinas as exhibited in the binocular perception of space.

In general both images are seen simultaneously and superposed in the field of vision. Yet in certain regions of the field there dominates now the one image now the other. Through an effort of attention either image may be made to extinguish the other. Yet the attention cannot long be kept fixed on either image without the other intruding itself. More especially the image forces itself into consciousness when it has a prominent and striking contour. It is a point in dispute whether two fields differently coloured ever yield a single composite sensation of colour (e.g., whether a blue and a red field yield the sensation of purple). Helmholtz and some others deny that this is the case, though there are not wanting good authorities on the other side. The perception of lustre which, as Dove has shown, may arise from the sterescopic combination of impressions of unequal light-intensity, as white and black, is an interesting instance of the coalescence of the impressions of corresponding points. Finally it is found that the colour-impression of one retina may be intensified by contrast with a simultaneous impression of the complementary tint in the other retina.

We may roughly gather up the results of these investigations into the nature of binocular vision as follows: (1) There are no points of the two retinas whose impressions are always and under all conditions indistinguishable. (2) In the normal and mature organ there are certain corresponding points or circles in the two retinas of which the impressions tend with more or less force, varying according to certain psychical conditions of the moment, to coalesce in single perceptions. How these facts have been variously interpreted, I shall try to show in another paper. The real meaning of the correspondence between the two eyes is a verata quæstio in the discussions of visual space. It is allowed 'by all that experience has something to do with the determination of the limits of single vision; but the point is sharply disputed whether this correspondence does not involve as well some connate anatomical connection which serves as a physical basis for a sort of à priori disposition to see single objects in a single JAMES SULLY.

space.

II. THE PHYSICAL BASIS OF MIND.

UNDER this title Mr. Lewes, in his new volume,* passes from the general part of his philosophical task to deal with the more special Problems of Life and Mind,' and delivers himself on various questions that have lately engrossed much attention. Prominent among these is the question of so-called Arimal Automatism, and it is proposed in the following pages to offer some remarks on the subject after considering his handling of it; but first it is necessary, as well as due to Mr. Lewes, to take account of other parts of the volume, which contain the results of long protracted inquiry.

In this country at least, Mr. Lewes holds an almost unique position. He is a philosophical thinker and psychological inquirer who is also a practical worker in physiology; or he is a physiologist whose positive investigations of the innermost phenomena of organic life are guided by trained psychological insight and an ever-present regard to philosophical principles. In either aspect of it, his activity is of prime interest to all who at this present time are concerned about the problems of Life and Mind. Physiological specialists, who naturally are every day more and more encroaching on the psychological domain, may draw much enlightenment from one who knows how to speak their language as well as the other; and psychologists, who have to endure many a sneer for their readiness to eke out subjective observation with second-hand objective discoveries, may repose special confidence in a fellow-inquirer who accepts ro physiological results that he does not himself verify. Those parts, therefore, of his present volume where he appears most distinctly in his double character of physiologist and psychologist, or prepares the way for assuming it, have the strongest claim on our attention here. A short preliminary survey of the volume will make plain what they are.

We have first a series of discussions on The Nature of Life'. Since it is animal organisms that manifest mind, a clear view of the distinctive character of vital organisation is naturally the primary requisite for understanding that special form of life which mind is. Towards the general argument of his volume, Mr. Lewes here more especially contends that no mechanical expression can ever adequately rep.esent the processes of life; he also impresses, for use later on, the very important distinction

*The Physical Basis of Mind, with illustrations. Being the Second Series of Problems of Life and Mind, by GEORGE HENRY LEWES. London: Trübner & Co., 1877. (Vol. I. of the First Series, The Foundations of a Creed, appeared in 1874, and Vol. II. in 1875.)

between Property and Function which he had the credit, nearly twenty years ago, of first bringing clearly into view in the physiological science of the present generation. The consideration of vital phenomena is then brought to a close in a long chapter on Evolution, which aims at showing that a struggle for existence is maintained not only among organisms but also among their component tissues and organs, and that the unity of type in organisms is rather to be explained by all-pervading laws of Organic Affinity than by Mr. Darwin's supposition of Unity of Descent. The next section is concerned with The Nervous Mechanism,' and contains much destructive criticism of current scientific doctrines, followed up by an exposition of such general notions of the structure and action of the nervous system as the author believes can be affirmed in the present imperfect state of knowledge. Then follows under the heading of Animal Automatism,' a somewhat varied collection of dissertations-historical, abstract, polemical-directed to the assertion of "the biological point of view" against a purely mechanical one in treating of mind as related to the living organism. And last, within the present volume, 'The Reflex Theory,' which forms so great a part of the prevalent doctrine of neuro-physiology, is subjected to an elaborate consideration from the same biological" point of view, taken as it had already been by the author in regard to this particular question when he wrote his well-known popular work The Physiology of Common Life.

The last two "problems," while intimately connected, arise naturally out of the "problem" of the Nervous Mechanism as treated by Mr. Lewes, and must be approached through it. On the other hand, the preliminary discussion on the Nature of Life, if its general import is kept in view later on, need not here detain us. Not the least interesting portion, it may only be remarked in passing, is that in which Mr. Lewes seeks to generalise the principle of Natural Selection by extending it to the organised elements of composite animal organisations; as he had already some years ago proposed to amend Mr. Darwin's theory in another direction, namely, by supposing Natural Selection to proceed upon an indefinite number of original protoplasts emerging under similar conditions, instead of the four or five or even one considered by Mr. Darwin himself at once necessary and sufficient to account for all the variety of related organic forms. Mr. Darwin, in reply to the earlier criticism, has admitted (Origin of Species, 6th ed., p. 425) the possibility that at the first commencement of life many different forms were evolved, but thinks it may be concluded that in that case only a very few have left modified descendants. One would gladly learn his opinion of the extension now proposed of his famous