As regards the distance at which man can distinguish objects, he is less gifted than many other animals; but in every other respect his visual powers are at least equal to that of inferior beings. We know very little of the sensations produced in animals by colours; it seems probable that they have a relative perception of them to a certain extent, as the sight of red irritates the bull, for example; and we know that birds of prey from a great height in the air distinguish the colour as well as the form of a lark or a quail hiding in the ploughed fields, although it so closely resembles that of the soil But if we should suppose them endowed with sensitive faculties, useless within the limits of their instinct, could we find anything in animals more perfect than the organs to which man owes the prodigies of painting? We must, however, distinguish here between that which pertains to the visual apparatus, and that which proceeds from the intellect The eye perceives the tints which nature offers in almost infinite variety; the mind compares them, and recognizes the elementary colours of which they are composed; the eye reflects in turn the model, the palette, and the picture; the mind perceives the relation of shades, and combines them in such a manner, that by mingling or contrasting them such a result is produced as conforms to the first impression; but in order that an artist may judge whether red or blue predominates in a violet tint, in order to appreciate the shade, the retina must transmit it to the brain in its purity.
At the manufactory of the Gobelins, we see the wools used in the fabrication of the tapestries arranged according to their shades. The number of these shades exceeds 28,000, and yet when we compare two approximate shades we distinguish them with facility, and perceive the interval which separates them.
The people who live in the country, seamen, and especially men living in a savage state, generally have sharper sight than the residents of cities. May not the habit of seeking to distinguish objects at a distance give the eyes a power which is not acquired when they always act within a limited horizon? Without assimilating exactly the effects of exercise on the eye to those which result from exercise of a muscle, we are justified in thinking that an almost incessant accommodation to great distances must influence the eye in that respect, and if, as is very probable, the accommodation takes place by the contraction of muscular fibres, the explanation of the increased range of the eye from exercise is very simple; but facts are wanting which verify and measure this increase in individuals. There is no doubt, however, that men from whom the horizon is habitually distant distinguish certain objects at a point where they are confused to other persons, although within the reach of their vision.
A ship appears on the horizon, a man unacquainted with the sea can hardly distinguish the sails of this white cloud springing from the waters; but a sailor will tell you that is a brig or a three-master, a war vessel or a merchant ship, and often he will even come at its tonnage, its lading, its nationality, and its name. The Arab and the European in the midst of the sands of Sahara see on the horizon an object, which to the European is only a black point without appreciable form; the Arab sees a camel distinctly, and declares that it is at such or such a distance, without ever being deceived.
The inexperienced mountain traveller sees before him a chaos of slopes and abrupt walls, of elevations and windings, among which he can distinguish neither route nor practicable passage; but the mountaineer sees at once the accessible points, and the turns which he must take to reach the summit of the apparently impassable barrier. This proves not that the sailor, the mountaineer, or the Arab have sharper sight than the stranger to their country; but that they have learned to know the signification of such and such details of form, such a particularity of colour and the like, which are for them distinguishing marks, which seem to trace before their eyes the description which they give to their fellow-voyager of objects that are either confused or imperceptible to him. It is therefore to acquired notions, and skill in seeing objects, rather than to extent of vision, that they owe the faculty of distinguishing objects at great distances.
We find also in all countries, and in all climates, men who have extraordinary powers of vision. Wrangel speaks in his voyage to the Polar seas, of a Yakoute who related having seen a great star swallow little ones, and then vomit them up again. That man, says Wrangel, had seen the eclipses of the satellites of Jupiter. Humboldt tells, in his Cosmos, of a tailor in Breslau, named Schcen, who also had seen the satellites of Jupiter with the naked eye. No examples of a greater range of vision are known.