nearly 200,000 miles. The German physiologist, Holmgren, and Professors Dewar and M'Kendrick have shown that this new influence, generated in the nerve by the agency of light, is accompanied by electrical development. Currents of electricity are produced whenever flashes of light are thrown upon the retina of recently killed frogs. But the nerve-influence is not merely a current of electricity, because this again travels with a speed which is measured by thousands of miles in a second, and not by hundreds of feet.

That the cerebral perception of a visual image is altogether a different affair from the mere stamping of a luminous impression upon a sensitive screen is further proved by a series of considerations that can be in no way explained by merely physical agency. Thus there are two eyes employed in the optical part of the process of vision, and two pictures are assuredly made upon the nerve-structures of those organs. But only one image is seen, unless when the consentaneous action of the two eyes has been abnormally deranged. There is an absolute and quite inseparable fusion of the two visual pictures into one mental impression or perception. This result, however, requires that each of the two images shall fall upon a duly correlated or corresponding part of the associated eyes. With squinting eyes this sympathetic correlation is deranged, and two images are seen. Then again, the images which are stamped upon the eyes are inverted, or upside down, as is manifest upon looking at them as they are formed within the eyes of a dead rabbit; yet the single image seen in the ordinary operation of sight is upright. The projections of solid objects traced in the pair of eyes are not absolutely the same. But, in the single picture which is seen, there is no confusion or incongruity; the two unlike projections are blended into the perception of an object standing out in solid relief. The explanation of all this intrinsically is that the optical images impressed upon the eyes are simply signs, and that these signs are interpreted by an ulterior operation in the brain.

The eye is supereminently, amongst the organs of sense, the one which ministers to the intellectual operations. It deals almost exclusively with matters of experience and comparison. The distance of objects that are looked at is inferred from the muscular effort which is made in augmenting the curvature of the crystalline lens of the eye, and in converging the two eyes upon the point of concentrated attention. The idea of actual magnitude is derived from the comparison of these efforts of accommodation and convergence with the size of the

impression upon the retina. The conception of a solid projection results from the consideration of the differences of aspect incident to varying points of view. These facts, and numerous other arguments of a like character, which exigency of space alone excludes from notice, all combine to demonstrate that vision is the work of prolonged and complicated experience and experiment which begins in the cradle, and only ends upon the margin of the grave. Helmholtz alludes very tellingly in his Popular Lectures' to the circumstance that vision and speech are alike in the peculiarity that they both deal with arbitrary signs which have to be learned before they can be understood. He says:

'Learning how to speak is obviously a much more difficult task than acquiring a foreign language in after life. First, the child has to guess that the sounds it hears are intended to be signs at all; next, the meaning of each separate sound must be found out by the same kind of induction as the meaning of the sensations of sight or touch; yet we see children by the end of their first year already understanding certain words and phrases, even if they are not yet able to repeat them. We may sometimes observe the same in dogs.'

and

'Now this connexion between names and objects, which demonstrably must be learnt, becomes just as firm and indestructible as that between sensations and the objects which produce them. We cannot help thinking of the usual signification of a word, even when it is used exceptionably in some other sense; we cannot help feeling the mental emotions which a fictitious narrative calls forth, even when we know that it is not true; just in the same way as we cannot get rid of the normal signification of the sensations produced by any illusion of the senses, even when we know that they are not real.'

'There is one other point of comparison which is worth notice. The elementary signs of language are only twenty-six letters, and yet what wonderfully varied meanings can we express and communicate by their combination! Consider, in comparison with this, the enormous number of elementary signs with which the machinery of sight is provided. We may take the number of fibres in the optic nerves as two hundred and fifty thousand. Each of these is capable of innumerable different degrees of sensation of one, two, or three primary colours. It follows that it is possible to construct an immeasurably greater number of combinations here than with the few letters which build up our words. Nor must we forget the extremely rapid changes of which the images of sight are capable. No wonder, then, if our senses speak to us in language which can express far more delicate distinctions and richer varieties than can be conveyed in words.'

The most recent, if not the most important, of the discoveries which science has made in reference to the structural arrangements of the eye is one which is not alluded to either by Professor Helmholtz or by Mr. Brudenell Carter, and

camera.

It is

which, in the first instance, seemed to indicate that the organ is, in reality, a photographic, as well as an optical, dark chamber. It has been long known that a peculiar colouring matter is deposited between the external protecting coat and the inner nerve-membrane of the eye. The intermediate layer with which this colouring matter is structurally associated contains also the delicate blood-vessels which are provided for the nourishment of the highly vital nerve-substance, and it has on this account been raised into the dignity of a special coat, called the choroid, or chorion-like,* tunic. The blood-vessels are distributed in this as minute radiating tufts which are intermeshed with each other, and between the interlacing vessels is laid down a flat pavement of hexagonal cells which are all densely packed inside with small opaque granules of a dark colour. This lining of dark pavement-like cells appears to answer the very important purpose of preventing the reflection and backward dispersion of light after it has struck upon the nerve-coat of the eye. analogous to the black stain of the inside of the photographer's The rod-like terminations of the retinal nerves, which have been already alluded to, abut immediately upon these pigment-cells, and are almost certainly connected with them by some intimate, although as yet not perfectly ascertained, relation. The German observer Boll, a few months since, observed that a very beautiful and quite distinctive purple colour is produced in the eyes of frogs in the immediate vicinity of these dark pigment-cells, and he further noticed that this purple colour was invariably bleached and destroyed on exposure to strong light, and that it was also capable of being reproduced out of the pigment-cells when the organ containing it was left for some time in darkness. The observations of Boll have since been amply confirmed by other experimenters, and the colour thus produced out of the pigment-granules in darkness has received by general consent the designation of visual purple.' The renewal of this delicate and evanescent tint can hardly be looked upon as a really vital act, because it occurs quite independently of any circulation of the blood. It can be destroyed and reproduced in the eye of a recently killed frog a considerable number of times by simply exposing the eye alternately to light and darkness. Another German experimenter, Kühne, has, however, ingeniously succeeded in fixing the image stamped luminously

The chorion is a well-known vascular membrane which bears a strong resemblance to the choroid coat of the eye.

upon the retina of a dead eye by washing the membrane, after exposure to light, with a solution of alum-potash. The idea not unnaturally occurred, after the discovery of this curious effect, that the production and destruction of this visual purple, and the reduction of its colouring principle by the influence of light, might have to do with the conversion of the optical impression into a conscious sensation-in other words, that vision may possibly be a photographic process. That such, however, is not the case, is manifest from another significant fact which further investigation has brought to light. Both the pigment-cells and the visual purple are absent altogether from the central pit, which is assuredly the seat of the most acute visual sensibility, and Kühne's photographic pictures accordingly cannot be produced there. The retinal cones, which are essentially the instruments whereby optical impressions are converted into visual sensation, are utterly destitute of all trace of colour. Kühne, indeed, seems to have already satisfied himself that frogs can see perfectly after all the visual purple in their eyes has been destroyed by long exposure to the action of light. It must therefore, for the present, be held that nothing conclusive is yet known as to the purpose for which this visual purple is formed, or as to the part it plays in the marvellous processes with which it is associated. The discoveries of Boll and Kühne are very curious, and well deserving of the further investigations which they will assuredly receive; but there is nothing in regard to them, so far as they have yet gone, which at all favours the assumption that a photographic theory of vision' is the goal to which the progress of science tends.

ART. IX.-Colonel Gordon in Central Africa, 1874-79. From Original Letters and Documents. Edited by GEORGE

BIRBECK HILL, D.C.L. London: 1881.

6

ALL the world has heard of Chinese Gordon, the young Major of Engineers, who having as a mere lad distinguished himself in the Crimea, and later on served gallantly in the war which ended with the capture of Pekin, in 1863 and 1864 led the ever victorious' army of the Chinese Emperor against the Taiping rebels and utterly routed them. Having accomplished this great feat he resigned his command and disbanded his army. For this signal service the Chinese Emperor made him a mandarin of a very high order,' giving him besides the rank of Ti-Tu, the highest in their

army, together with the four suits of uniform proper to his position, which no doubt were most acceptable after the wear and tear of the Taiping war. The English Government, with whose approval Gordon's services had been rendered, was, says Mr. Hill, more moderate in its rewards.' By it he was made a Lieutenant-Colonel and a Companion of the Bath, and with these and the reputation of a distinguished general and leader of men-renowned, as a young German prince who served under him said, as a bright example of a Christian soldier'— he left China. On his return to England in February 1865, Colonel Gordon was appointed Commanding Officer of Engineers at Gravesend, and employed for the next six years on the erection of fortifications to defend the mouth of the Thames. At the end of 1871 he returned to the East, where his military career had begun, as English Commissioner on the European Commission of the Danube.

This great capacity and energetic will were not, however, destined to be ingloriously wasted in dredging the muddy mouth of the Danube and in levying dues on the shipping which passes through it. In September 1872, fate, as we feel sure Colonel Gordon would call it, threw him in the way of Nubar Pasha, whom Mr. Hill calls the famous Egyptian Minister,' then at the height of his power, and who now, after many reverses, appears likely to be still more powerful in the present Egyptian crisis. Be that as it may, Colonel Gordon and Nubar met at Constantinople. The Egyptian Pasha was seeking for a man to succeed Sir Samuel Baker, whose fighting government of the Upper Nile basin was about to expire. To make a long story short, the reversion to Baker was offered to Gordon, and ultimately accepted by him, again with the approval of the British Government. In an interview with the then Khedive at the end of 1873, Gordon was told to fix his own terms as to salary. He took the modest sum of 2,000l. a year, and was graciously furnished with final instructions from the Egyptian ruler, of which all we shall say here is that they read remarkably well on paper, the main points being that the Khedive declared his utter abhorrence of the slavetrade, which he was determined to put down in Equatorial Africa by forming the Upper Nile province into a separate government under Gordon, and by claiming as a monopoly of the State the whole of the trade with the outside world. The reader will see even from this brief description of Gordon's instructions that the ex-Khedive, excellent in his intentions no doubt, had yet a shrewd eye to the main chance. It is instructive to note how Egypt has gradually invaded Central Africa. In