Some persons are gifted, as it were, intui tively with the power of judging correctly of the true dimensions of objects. Napoleon possessed this in an eminent degree. He could tell at a glance the number of men com posing a distant mass of troops, and the space of ground they would cover when deployed. In doing this a mental computation must have taken place unconsciously, rapid as thought, and based on a combination of great powers of calculation, long experience, and profound knowledge of the subject.
For the production of a distinct image on the retina it is necessary that it be of a certain magnitude, which will depend on the suscep tibility of the eye. We may here observe that the apparent magnitude of an object must not be confounded with its apparent superficial magnitude, the term being applied to its linear magnitude. The apparent superficial magni tude varies in proportion to the squares of the apparent magnitude. The image of an object moderately illuminated must be 0.001 of an inch long, or the extreme rays of light must form an angle of half a minute in the eye at a minimum ; whence it follows that an object of mean illuminating power will be visible if i:s distance is not greater than 68,000 or 69,000 times its greatest length. Strongly luminous bodies, as the fixed stars, are visible at infi nitely small visual angles. They excite in the eye merely a sensation of light, without creat ing any impression as to their apparent mag nitude. The disk of the moon subtends a visual angle of half a degree, the diameter of its picture on the retina will be Tiu of an inch, and the entire superficial magnitude of the image the of a square inch ; yet forms of light and shade are perceptible whose linear dimensions occupy upon the re tina a space whose diameter does not exceed the of a square inch. The eye in a healthy condition is capable of reading print in the light of the full moon, and that of the noon day sun, their intensities being to each other as I to 300,000. Plateau asserts that white may be distinctly seen in the light of the sun at an angle of 12", yellow at an angle of 13", red at 23", and blue at 26", but that in ordinary day light these angles must be half as large again.
In estimating the motions of objects, we are guided by the movement of their images on the retina; and unless a body moves in such a manner that the line of vision shall describe at least one degree in each minute of time, its motion will not be perceptible ; for which reason we are not conscious of the move ments of the heavenly bodies. The more nearly at right angles to the line of vision the direction of the motion-is, the greater will be the apparent motion produced by any real Movement of an object.
Erect Vision. —A variety of explanations have been offered to account for objects not being seen reversed, according to the position in which they are depicted on -the retina ; but it would appear that by many a sufficient distinction has not been drawn between seeing the image and seeing by means of it. A little reflection shows that the actual perception of the object takes place in the sensorium, and that the image on the retina is only a necessary step in the pro cess. In truth, we have no notion of upright or inverted, except that which is founded on experience. A man is upright whose head is upwards, and his feet downwards. Whatever be our standard of up or down, the sensible representation of up will always be an image moving on the retina towards the lower side, and the sensible representation of down will be a motion towards the upper side. The head of the man's image is towards the image of the sky ; its feet are towards the image of the ground ; and consequently it cannot ap pear otherwise than upright. So, as all objects are inverted on our retinae, they do not change their relation one with another, and our only knowledge of position is from relative rela tion, therefore we may truly say that we do not see the image on the retina, but by means of it. For every image on the retina, we sub stitute an object, and seek in a definite direc tion for the object corresponding to a definite image on the retina. In this we are assisted by other perceptions of sense, there existing the greatest harmony between such percep tions in respect to locality.
Dr. Alison, in an able paper*, advances the opinion, that the harmony between the inti mations acquired by sight and by touch, as to the relative position of objects or their parts, notwithstanding that the impressions made by them on the external organs of sight and of touch are arranged inversely in regard to one another, arises from the course of the optic nerves and tractus optici, whereby im pressions on the upper part of the retina are in fact impressions of the lower part of the op tic lobes—that is to say, of the sensorium and impressions on the outer part of the retina are, in like manner, on the inner part of the sensorium. This theory was first suggested by Mr. Dick, a veterinary surgeon ; but though ingenious, can hardly be considered satisfac tory, as it implies the necessity for con ditions which cannot always be fulfilled : and truly the question is of a nature not to be decided merely by anatomical inquiry.