purpose, we may regard the uncut pencils as representing upon an enormously magnified scale the rods of the retina, and the pointed ones the cones.

The flat upper ends of the pencils may be painted in different uniform colours, and arranged so as to form a large picture in mosaic, and if this is looked at from such a distance that its image on the retina is a tenth of an inch square (which will be the case when the picture is about forty yards away) all possibility of distinguishing the separate elements which compose it will be lost, and the picture will seem to be a perfectly continuous one.

Although the light which enters the eye cannot reach the rods and cones until it has traversed all the other layers of the retina, yet these intervening layers, being transparent, offer little obstruction to its passage, and it can hardly be doubted that the rods and cones are the special organs upon which light exerts its action, the picture focussed upon their ends being in truth an exceedingly fine mosaic.

From every separate element of the mosaic—from every single rod and cone—there proceeds a slender transparent filament: all these make their way through the intermediate layers of the retina, without, as is believed, any break of functional continuity, and emerge near its internal surface; here they bend over at right angles, and the thousands of filaments form a tangle which lines the inside of the eye like a fine network, and constitute the layer of optic nerve-fibres already referred to.

The filaments, or nerve-fibres, do not however terminate within the eye; they all pass through the hole marked N in the figure, and thence, in the form of a many-stranded cable, constituting the optic nerve, they are led to the brain, to which each individual fibre is separately attached. If, therefore, what I have said is true—and, though it has not, I believe, been all rigorously proved, yet the evidence in its support is exceedingly cogent—it follows that every one of the multitude of rods and cones has its own independent line of communication with the brain. The mind, which is mysteriously connected with the brain, is thus afforded the means of localising all the points of luminous excitation relatively to one another, and furnished with data for estimating the form of the object from which the light proceeds.

There are two small regions of the retina which are of special interest. One of them lies just over the opening N where the optic nerve enters. Here it is evident that there can be no rods and cones, their place being wholly occupied by strands of nerve-fibre. Now it is remarkable that this spot is totally insensitive to light.

The other interesting portion is situated opposite the middle of the front opening, and is marked by a small yellow patch, in the centre of which is a depression or pit, which is shown in an exaggerated form at F, and is called the fovea. It has been ascertained that the depression is due partly to the absence of the layer of nerve-fibres, which are here bent aside out of their natural course, and partly to a local reduction in the thickness of some of the intermediate retinal layers. This spot, being at the centre of the field of vision, occupies a position of great importance, and the evident purpose of the superficial depression is to allow the light to reach the underlying bacillary layer with as little obstruction as possible. It is noteworthy that the bacillary layer beneath the yellow spot is composed entirely of cones, the rods, which elsewhere are in excess, being altogether wanting.

The only other accessory of the visual apparatus to which I shall refer is the iris (I I, Fig. 2), a coloured disk having a central perforation. This can be seen through the cornea and is consequently a very familiar object. The iris serves the same purpose as the stop, or diaphragm, of a photographic lens, its function being to limit and regulate the quantity of light which is admitted into the eye. The size of the central opening, or pupil, varies automatically with the intensity of the illumination: in a strong light the opening becomes small; in a feeble light or in darkness it is enlarged. The pupil also contracts when the eye is focussed upon a near object and dilates when the vision is directed to a distance.

This brief sketch may serve to give some slight idea of the complexity and delicacy of the visual apparatus. Only a few of its more salient features have been touched upon; when our scrutiny is carried into details the complexity becomes bewildering. Even such simple-looking things as the cornea and the vitreous body turn out on close examination to be most elaborately constituted. Much, no doubt, remains to be discovered, and of what has already been investigated much is at present only partially understood.

And yet, though it is true that man is “fearfully and wonderfully made,” it is equally true that he is far from perfect; and while there is no structure in the whole human anatomy which exhibits so abundant a profusion of marvels as the eye, there is perhaps none which is marked with imperfections so striking.

Many of its defects are the more striking because they are so obvious, being such as would never be tolerated in optical instruments of human manufacture. In any fairly good camera or telescope or microscope we should expect to find that the lenses were symmetrically figured, free from striæ and properly centred; also that they were achromatic and efficiently corrected for spherical aberration. In the eye not one of these elementary requirements is fulfilled.

The external surface of the lens formed by the aqueous humour and the cornea is not a surface of revolution, such as would be fashioned by a turning lathe or a lens-grinding machine; its curvature is greater in a vertical than in a horizontal direction, and the distinctness of the focussed image is consequently impaired. Again, the crystalline lens is constructed of a number of separate portions which are imperfectly joined together. Striæ occur along the junctions, and the light which traverses them, instead of being uniformly refracted, is scattered irregularly. Moreover the system of lenses is not centred upon a common axis; neither is it achromatic, while the means employed for correcting spherical aberration are inadequate. The purchaser of an optical instrument which turned out to have such faults as these would certainly, as the late Professor Helmholtz remarked, be justified in returning it to the maker and blaming him severely for his carelessness.

I would not, of course, have it believed that scientific men are conceited enough to imagine themselves capable of designing a better eye than is to be found in nature. That would be an absurdity. They are quite ready to admit that there may exist sufficiently good reasons for the undoubted blemishes which have been indicated, as well as for others which will be referred to later. It is indeed well known that the general efficiency of a machine as a whole may often be best secured by the sacrifice of ideal perfection in some of its parts.

With all its anomalies the eye fulfils its proper function very perfectly, and is regarded by those who have studied it most closely with feelings of wonder and humble admiration.