feet, however, the rays of light are already so little divergent, that they are accounted parallel; whilst at a less distance than eighteen feet the rays are called divergent. The course of parallel rays is as follows (fig. 17): reflected from a, the rays pass to and through the cornea, where they are bent or reflected towards the axis of the eye (b, c); they then pass through the aqueous humour to the crystalline lens, where they are still more refracted, so as to cross the axis of the eye, and, traversing the vitreous body, terminate on the retina, where an inverted image of the object is formed.

If any divergent rays from a near object were to pass through the eye, whilst the eye was in the same state as that in which the parallel or distant rays come to a focus, the effect would be that the picture of the object would form behind the retina, and thus perfect and well-defined sight would be impossible for near objects. To meet this there is a little mechanical arrangement in the interior of the eye. The ciliary muscle (Plate XI., fig. 3, h), attached in front to the sclerotic, and behind to the ciliary processes, when in action (that is, when the eye is adjusted for near objects), pulls forward the ciliary processes, and therefore the plaitings of the hyaloid membrane; and by means of the attachment of the hyaloid to the circumference of the lens, this body is also brought forward; the canal of Petit prevents the muscle acting on the vitreous body, by leaving a space between. By this means a greater distance is produced between the lens and the retina, and thus the image of the object is brought to a focus directly upon the retina.

There is nothing that man can construct to equal, in beauty of mechanism, the iris. It is a curtain, circular in shape, and with an aperture in the middle, which can either contract or dilate without showing the least puckering or plaiting of the surface of the iris. The contraction of the pupil takes place in a strong light, and is intended to exclude an excess that would act injuriously on the retina; the dilatation takes place in dull light or darkness, for the purpose of allowing every possible ray to pass.

During adjustment of the eye for a near object, it is turned inwards, and the same nerve by which this movement is effected causes contraction of the pupil. This contraction probably merely excludes excess of light, and does not aid adjustment of the eye; as adjustment has been observed to be perfect even when the iris was entirely absent.*

* For much interesting and valuable information respecting the eye, see the second volume of Todd & Bowman's "Physiological Anatomy;" Griffiths & Henfrey's "Micrographical Dictionary" (Van Voorst), article "Eye"), and the books of reference there named.

EXPLANATION OF PLATE XI. AND WOODCUTS.

Fig. 1. The eye of the ox, seen laterally, the muscles having been removed. a, the pupil of the eye; b to b, the cornea; c, c, and d, ends of muscles; e, blood-vessels entering the eye: c, d, and e, are on the sclerotic; f, the optic nerve.

a, the

Fig. 2. The same; the cornea and sclerotic have been removed. crystalline lens, seen through the pupil; the band, c to c, is the ciliary ligament and muscle; b, the iris; e, the choroid, showing its vessels; f, the optic nerve.

Fig. 3. A diagrammatic section of the eye, enlarged. a, the cornea, consisting of anterior elastic lamina, cornea proper, and posterior elastic lamina; b to b, by a, shows extent of cornea; c, c, c, the sclerotic; d, the anterior chamber of the eye; e, the pupil; f,f, the iris; g, g, the ciliary processes; h, h, ciliary ligament and muscle; i, the crystalline lens, surrounded by its capsule, j; k, k, the canal of Petit; shows the termination of the retina (the scarlet line); the black line between the retina and the sclerotic is the choroid; the light line beyond the retina is the hyaloid membrane, which passes on to the lens; m, the vitreous body; n, the optic nerve; o, the central artery of the retina.

Fig. 4. Fibres of the sclerotica.

Fig. 5. Epithelium of the choroid. a, from the dark part in front; b, over

the tapetum.

Fig. 6. a, detached vesicles; b, detached granules, from the retina.

Fig. 7. Fibrous layer of retina; a, a blood-vessel.

Fig. 8. The iris, seen in front; a, the pupil; b, the iris.

Fig. 9. The iris and ciliary processes, seen from behind; a, the pupil; b, the iris, partly hidden by c, the ciliary processes.

Fig. 10. Transverse section of the cornea proper.

Fig. 11. Section of anterior elastic lamina. a, cornea proper; b, epithelium;

c, elastic lamina.

Fig. 12. Jacob's membrane. b, when seen from above; a, appearance of detached particles.

Fig. 13. Section of retina. a, Jacob's membrane; b, granular; o, vesicular; d, fibrous layer: along the latter is seen passing a blood-vessel. Fig. 14. The crystalline lens, seen in front.

Fig. 15. The lens, separating into three divisions, and showing its laminated

structure.

Fig. 16. Fibre of the crystalline lens (Todd and Bowman).

Fig. 17. Diagram to show the inverted image produced on the retina; b, c, the

optic axis.

Figs. 4, 5, 6, 7, 10, 11, 12, and 13 are reduced from the microscopic appearances at 250 diameters, drawn by the camera lucida.

MARS.

BY JAMES BREEN, F.R.A.S.

OF

F all the planets of the system, MARS, which for the last few months has been shining so brightly in the heavens, is that whose topographical details are best known to us. By glancing over the accompanying pictures of its telescopic aspect, we immediately perceive how much is revealed of its surface,of its islands, continents, seas, and snows, by means of powerful optical aid. It is the planet which most strongly resembles the Earth in the duration of its days and seasons; the existence of an atmosphere is everywhere apparent: being proved by the dimness of the dark streaks and spots at the circumference, as compared with their distinctness at the centre of the planet (for at the former the solar light has to penetrate through a dense stratum of air, and is again refracted through the same thick medium);— by occasional clouds passing over its surface;-by the snowzones piled up and stretching over vast spaces at its poles in the winter, which melt away gradually as the Sun ascends above the horizon in the summer, and dissipates the frost and darkness which for months previously had reigned in those arctic and antarctic regions. In the planet Jupiter a small telescope may more readily show the dark belts and spots; but those are ever changing and drifting about with variable velocity; the terra firma is scarcely perceived on this immense body, which appears to have an economy of its own, hidden from us by great masses of cloud, through an occasional break of which we perhaps sometimes catch a glimpse of the dark body of the planet. On the surface of Mars, on the contrary, the dark spots preserve the same position and relative dimensions, and we appear to be looking at a miniature globe pencilled over with dim seas and continents. From year to year the sea does not appear to encroach upon the land, nor the land upon the ocean; all the changes which are perceived are purely meteorological-the presence of clouds and murky weather, and snow during the winter,-of a clear atmosphere and sunny clime throughout the summer.

The first circumstance we detect in looking at the planet Mars, is its exceedingly red light, which is quite different from

that of the other bodies that circulate about the Sun. This does not appear so prominently, however, when looked at with a telescope, as when seen by unaided vision. Still, however, even with the former, the orange light is very decided, and if compared with the Moon or a neighbouring white star, the contrast is sufficiently striking. Viewed when the whole disc of the planet is illuminated, its form appears quite circular, and no suspicion is aroused of a flattening at the poles, or bulging forth of the equatorial regions. But when the micrometer is applied, and careful measurements are made of its polar and equatorial diameters, several observers have agreed that there is a slight variation from the circular form, although the results which they have obtained are very discordant. Herschel was the first who suspected the elliptical form of the planet, and who patiently set about to determine the amount of this variation. To arrive at a knowledge of the figure of the Earth requires long and arduous labour; but in the case of the planets the method is more simple, and the diameters at different parts of the disc may be said to be measured with the same facility as if it were a palpable object. Herschel found that the proportions of the equatorial diameters of Mars were as 1355 to 1272, or near as 16 to 15. Schroeter could not perceive any such ellipticity, and was of opinion that the two diameters were in the proportion of 81 to 80. Arago found them to vary in the proportion of 31 to 30. The Greenwich observations of late years give this variation as 52 to 51, and as 62 to 61. Other observers, among whom is Bessel, have not been able to detect the slightest difference between the diameters. Herschel, however, states, that on one occasion he showed the planet to some scientific friends, one of whom considered that it was as considerably bulged out at the equator as the globe of Jupiter. At certain times, as the whole surface of Mars is not illuminated, it will appear of the same figure as the Moon when three or four days before or after full; but even when this was the case, the flattening at the poles was still readily perceived by Herschel. There are a few circumstances which militate against the correctness of those measures; sometimes the white cap of snow seems to project over the edge of the planet, at others the equatorial margins are exceedingly bright and radiating. According to theory, the proportion of the polar to the equatorial zone should be as 192 to 193; but different degrees of density at various parts of the globe would, of course, alter this. By observations made in September and October of 1862, Mr. Main concludes its polar diameter to be 4,221 miles, and its equatorial 4,332 miles,-a great difference for a small body like Mars, which has almost the same density as the Earth. In the latter body, the difference between the

polar and equatorial diameters amounts only to twenty-six miles; these quantities being respectively 7,899 and 7,905 miles.

Be the planet circular or elliptical in figure, however, it is easy to see that it rotates on its axis, and that too in about the same time as Mercury, Venus, and the Earth. In other respects, also, the similarity is striking, these planets being nearly of the same size, globular figure and density, and differing greatly from the huge exterior planets in those respects. The rotation of Mars is readily perceived by watching it from hour to hour throughout the evening, when it will be seen that the dark spots pass over the disc from west to east in the same sense in which the Earth is rotating. In about twenty-four hours and thirty-seven minutes, or on the following evening, the spot which was first observed will be found to have returned exactly to the same position, and the others will follow in the same succession as on the previous day, the polar snow-spot retaining nearly the same place. By further watching, the patient observer will find that the equator of Mars is only inclined a few degrees more to the plane of its orbit than that of the Earth, and that, consequently, their seasons are about the same. From the longer year of Mars, however, the interval during which the polar regions are hidden from and exposed to the rays of the Sun, is very different. At the latitude of the British islands, the shortest day on Mars is only about six hours, whilst on the Earth it is between seven and eight. The longest day on Mars, at the same latitude, would be nineteen hours, whilst on the Earth it is only seventeen hours. At seventy degrees of north latitude the Sun remains above the horizon for sixty-nine days, whilst on Mars at the same latitude it is above the horizon for 169 days. The pole of Mars is exposed to the Sun for 338 days, and hidden from it for the same time; whilst on the Earth the polar day or night is only 180 days.

Some observers have complained of the striking monotony and uniformity of the surface of Mars. If we cast a glance at the accompanying pictures of the planet, we shall hardly be disposed to indorse this opinion. On the contrary, the variety of scenery is deeply interesting. The seas, or dark portions, are remarkably sinuous in their course; the indentation of the coast, caused by bays and creeks, is very picturesque. It is true that mountains and vales, such as those on the arid surface of the Moon, cannot be perceived; but by attentive watching, it will be seen that the bright portion of the planet is curiously dotted over with a mottled ground, and that the dark seas vary greatly in the intensity of their tint, even when the spot is at the centre of the planet, and is viewed most favourably. It