individual to appreciate figures projected in relief, as shown by the ingenious experiment of Wheatstone.

9

Of the various theories that have been propounded, in explanation of the phenomenon of single perception from dual vision, I will direct attention to those three only which have commanded the greatest amount of respect, both by their ingenuity, and the deservedly high character of their authors. Sir Isaac Newton asks the question:

"Are not the species of objects seen with both eyes united, where the optic nerves meet before they come into the brain, the fibres on the right side of both nerves uniting there, and, after union, going thence into the brain in the nerve which is on the right side of the head; and the fibres on the left side of both nerves uniting in the same place, and, after union, going into the brain in the nerve which is on the left side of the head; and these two nerves meeting in the brain in such manner that their fibres make but one entire species or picture, half of which on the right side of the sensorium comes from the right side of both eyes, through the right side of both optic nerves, to the place where the nerves meet, and from thence on the right side of the head into the brain; and the other half, on the left side of the sensorium, comes in like manner from the left side of both eyes.

"The optic nerves of such animals as look the same way with both eyes (as of men, sheep, dogs, oxen, etc.) meet before they come into the brain; but the optic nerves of such animals as do not look the same way with both eyes (as of fishes and the cameleon) do not meet".

This explanation, though simple and apparently satisfactory at first sight, will not stand the test of close inquiry; an opinion which appears to have been shared by its great author, from the circumstance of his having offered it in an interrogative form. It proves too much and too little for the intended purpose. When an object is viewed with both eyes, it is not the half of a single image which is depicted on the corresponding sides of the two retina, as stated by Newton, but the whole; and if this be not his meaning, but rather that the half of a double image―i.e., the whole of a single one-is portrayed on each retina, then the original difficulty remains untouched.

Again, if an object, placed directly before the face in the common axis, be not of such dimensions as to occupy a space greater than the interval between the optic axes at the points of their emergence from the cornea-if, in short, it be a mere point, it will obviously not be depicted upon "the right side of both eyes", and thence propagate an impression to the sensorium through the right side of both optic nerves", nor "upon the

66

Philosophical Transactions, 1838, part ii. p. 371, et sequent. 10 Newton's Optics, query 15.

left side of both eyes", and thence to the sensorium "through the left side of both optic nerves", nor indeed upon any points whatever of the retina, save those corresponding to the optic axes or their immediate vicinity, unless the axes intersect before reaching the object, in which case a double perception would be the result, as shown by the second experiment.

Nor does the anatomy of the optic chiasma, on which Newton relied for confirmation of his theory, afford it more than a negative support, for although in no animal, as far as we know, capable of viewing an object with both eyes simultaneously, is there an absence of an optic commissure, still in many whose eyes have a decidedly lateral aspect, and are therefore incapable of axial convergence, it exists in a not less perfect state than in the former. This is the case in most of the whale tribe, and in birds generally.

The opinion of Müller" on this subject will appear from the following quotation :—

"Parts of the retina which lie in the same segments of the sphere, in the same meridian and the same parallel of latitude, the middle point of the retina being regarded as the pole, or which lie at equal distances in the same direction from the centre of the retina, are completely identical. All other parts of the retina are non-identical, and when they are excited to action the effect is the same as if the impressions were made on different parts of the same retina. If the position of the eyes with regard to a luminous object be such that similar images of the same object fall on identical parts of the two retinæ, the object cannot be seen otherwise than single; but in any other case two images must be seen".

This theory of" identical" and "non-identical" parts affords a satisfactory explanation of many of the phenomena of vision, but not of all. In the second experiment it has been shown that if two bodies placed in the common axis be brought into such close proximity, that whilst one of them is in the optic axes, the lines of visible direction of the other shall intersect those axes at a retinal angle of less than 3°, the latter will produce but a single perception in the sensorium, although its image is depicted on the inner, or the outer side of the axis, i.e., on non-identical parts of both retinæ.

The last theory I shall notice is probably the most beautiful that has been offered on this subject, and, though insufficient in itself to meet all the requirements of the case, is nevertheless, in its measure, a satisfactory explanation, namely, that of Aguilonius, as adopted and modified by Porterfield.

12

"Müller's Physiology, by Bayly, vol. ii. p. 119.

13 Aquilonii Optica, as quoted by Wells, opus citat.

The

theory of Aguilonius may be stated thus:-All bodies seen at a glance appear situated in the plane of the horopter; if really situated in that plane, they appear single when viewed with both eyes; if not really situated in it, they appear double; because, if situated behind the plane of the horopter, their lines of visible direction intersect each other before reaching that point of it in which they appear situated, and, if in front of that plane, they appear likewise double, because their lines of visible direction pass through it at different points before intersection.

Porterfield writes:13_

"In seeing objects, the mind, by means of an original and connate law, to which it has always been subjected, traces back its own perceptions not only from the sensorium to the retina, but from thence also outwards towards the object itself, along right lines drawn perpendicularly to the retina, from every point of it on which any impression is made by the rays forming the picture, by which means the mind or visive faculty does always see every point of the object, not in the sensorium or retina, but without the eye, in these perpendicular lines".

These two theories combined afford a rational explanation of single vision from a double image, and of double vision from a single object, notwithstanding the sneer of Sir C. Bell," but they do not assign to the retina its due influence in these phenomena, much less attribute to special portions of it special functions, which it has been the purpose of this article to establish, and without admitting which, I hold it to be impossible to account for many visual phenomena, as, for example, the fusion of the two images of the included body which results from approximating it to the axial body, as in the second experiment.

13 A Treatise on the Eye. Edinburgh, 1759, vol. i. p. 372. 14 Philosophical Transactions, loco citat.

491

SCIENTIFIC NOTICES.

PHYSICS.

1.—The Rev. Dr. Lloyd on the Direct Magnetic Influence of a Distant Luminary upon the Diurnal Variations of the Magnetic Force at the Earth's Surface.

The diurnal changes of terrestrial magnetic force, which exhibit phænomena inexplicable by the theory of thermo-electric currents, have led Dr. Lloyd to examine the influence which might be ascribed to the sun and moon, on the hypothesis that these bodies are endowed with inherent or induced magnetism.

Having found analytical expressions for the three rectangular components of the magnetic forces upon any point at the earth's surface, Dr. Lloyd resolves these components in the direction of the earth's radius and in the tangent to the meridian, and thus finds, for the variations of U and V, the horizontal and vertical components of the earth's magnetic force, the equations

sin 0 (2P sin d+Q sin λ cos d)+cos (2P sin λ cos d

-Q sin 8)-R cos λ cos

{sin

0

= {(2P cos 0+Q sin 0) cos

and

where D represents the distance of either luminary from the centre of the earth, the latitude of the point on the earth's surface, the angle included between its meridian and that of the magnetic luminary, and the magnetic declination, P, Q, and R, the sums of the components of the magnetic forces in the direction of the rectangular axes.

Hence it would follow, that a diurnal inequality having one maximum and one minimum, should take place in the magnetic intensity depending on 0, or in other words, on the hour angle of the sun or moon. The actual phenomena are, however, entirely opposed to this. Hence the author concludes, that the phenomena of diurnal variation are not caused by the direct magnetic action of the sun and moon. This inquiry is valuable in so far as it seems to remove from the theory of terrestrial magnetism, a supposition which might otherwise stand in the way of the true path of investigation.-Proceedings of the Royal Irish Academy, February, 1858, and Phil. Mag. for March, 1858.

2.-On the best Unit of Length.

This inquiry is developed in a group of systematic replies by Mr. James Yates, to a series of questions circulated by the International Association as to the determination of a unit of length best adapted to the present wants of mankind. The result of the whole inquiry is an overwhelming mass of the clearest evidence in favour of the metre, and the conclusions have been embodied in a report which has been unanimously adopted by the Association.

Mr. Yates has taken pains to show the great difficulty with which the question of improved weights and measures had so long to contend in England, in spite of the exertions of intelligent and energetic advocates. Amongst others he says " "None deserves more honourable mention than Sir John Riggs Miller, an Irish baronet, who, in order that he might collect information and promote reform, sat in parliament for one of the lowest of the rotten boroughs, received more than a thousand letters, obtained the appointment of a committee to investigate the actual state of the weights and measures used throughout England, and exposed their faults in two speeches, more clear, powerful, and conclusive than any others upon the subject. His efforts were abruptly terminated by the dissolution of parliament, so that England remains in the same disgraceful condition which he so eloquently and ably exposed".

Mr. Yates shows clearly that the hair-splitting objections against the metre arising from its presumed want of accuracy as an exact representation of the ten-millioneth of a quadrant of the meridian, are altogether devoid of any practical application. He points out its superiority as a standard, over its competitor, the length of the second's pendulum, and shows that it is susceptible of being applied advantageously by its subdivisions and multiples to all purposes in the arts and in trade where exact measurements are desirable. Besides showing the progress which its application is making in foreign countries, he shows that even in England it has already obtained a footing partly among men of science, and partly among manufacturers and traders, whose operations are connected

What is the best unit of length?-An inquiry addressed to the International Association for obtaining a uniform decimal system of measures, weights, and coins. London: Bell and Daldy.