It is stated that the abnormal sensations of colour are so intimately connected with these objects, that some can only he seen without them by a strong mental effort, and that in the case of others this does not suffice.
In the present state of our knowledge we are not in a position to offer any satisfactory explanation of this singular anomaly of vision. That its seat is not in the eye but in the sensorium is however most probable, and in this opinion Dr. Wartmann concurs. More extended opportunities for observations will doubtless throw additional light on what must be regarded as a very curious subject.
Anorthopia (lc, not; OpOtry, straight ; &pa, vi sion).— This is a condition of vision far from uncommon, and is characterised by the in dividuals subject to it being unable to discern when objects are not to the other, and is often accompanied by a want of ability to distinguish whether objects are symme trical. Such persons are incapable of drawing objects correctly; a house will be sketched with its proportions wrong and leaning on one side, and a figure will be equally unna tural, yet the artist wi I be sublimely un conscious of any defects. They are unable to discern whether pictures are straight on the walls, or blinds drawn parallel with the window frame ; Negroes are very subject to this _peculiarity of vision. Nothing is more common than to see them, when mark ing out the ground-plan of a house, path, or boundary wall, draw the lines as awry as possible, and yet persist that they are quite straight, nor can they be convinced to the contrary. It has appeared to us that the persons in whom this condition of vision existed in a marked degree, were charac terised by unsymmetrical heads and faces, but this may have been a coincidence merely.
In children who show evidences of anor thopia, pains should be taken to overcome it by practice and tuition. They should write upon ruled paper at first, and when subse quently writing on ordinary paper, should always be made to place it straight before them and to write across it by the hand moving on the wrist which should be a fixed point, and seldom moved. In drawing, thef,! correctness of the lines should be ascer tained by admeasurement ; and the study of geometry, perspective, and all other branches requiring attention to symmetry are calculated to be of service.
Myopia (1.,6co, I shut ; 4, the eye), commonly called near sight, is an affection almost, if not entirely, confined to civilization.
"-Every eye, when in a state of repose, is adapted by its size, figure, and the refrac tive powers of its media, to the formation of a distinct image of an object presented before it, at one particular distance. This differs in different individuals ; but from 12 to 20 inches may be regarded as the distance at which ordinary print is legible, the shortest distance at which it can be seen clearly and without! exertion being from 6 to 8 inches. A person:
who brings small objects nearer to the eyes. than 6 inches is considered myopic. • It is commonly supposed that a certain visible conformation of the eye exists in con nection with, and characterises myopia ; that the eye is full, the cornea prominent, the an terior chamber large, &c. : but this is by no means necessarily the case; for it may happen that of two individuals in whom the apparent configuration of the eyes is the same, one will be myopic, the other presbyopic. It is also a popular belief that near-sight decreases with age; such does not accord with our experi ence. We have repeatedly seen persons above sixty years of age as near-sighted as in their youth, and are acquainted with a lady of eighty-five, who has certainly never fatigued her eyes with work or reading, and who still uses, and has used for many years, a No. 7. myopic glass.
Myopia may arise from imperfection in the form, consistence, or relation of some of the refractive media ; or it may result from the loss of adjusting power. The first description may be induced by too great convexity either of the cornea or lens, or both ; by an undue density of any of the refractive media ; by a too great distance between the cornea and retina, &c. The effect is always the same, namely, to cause the rays of light to be so much refracted that, in place of being concen trated to a focus exactly on the retina —a point essential to distinct vision—they are brought to a focus before reaching it. It is a law in optics that the nearer an object is brought to the eye, the more divergent are the rays proceeding from it, and consequently the greater the distance at which they will be collected in a focus by refractive media. For this reason it is that near-sighted persons habitually hold objects very close is their eyes, as by that means the image is thrown hack upon the retina. One of the causes of imperfect vision in such persons, is the circle of dissipation formed on the retina by the rays after they have crossed at the focal point. This is principally produced by the circum ferential rays; and if these can be excluded, vision is rendered much more distinct. Thus it is that great assistance is afforded by look. ing through a pin-hole in a card, and even by holding to the eye a hollow roll of paper, or the hand partially closed. The habit of half shutting the lids which has given the name to the affection, has reference to the same object—the exclusion of the circum ferential rays.