ODDITIES OF VISION.

Star (Christchurch), Issue 5567, 16 May 1896, Page 1

ODDITIES OF VISION.

While the eyesight of many people is defective, a " still greater number have eyes which are unlike each other in seeing power. In fact, many practically make use of one eye only ; the other, having been originally a little defective, was by degrees, called on for duty less and less, and accordingly degenerated. One gentleman v is reported who could see, with his light eye, the figures on the face of a clock fiveeighths of an inch high at 12ft, while he could see them as clearly with, the left eye only at eight inches distance, and, curiously enough, this extraordinary difference in the powers of vision of both eyes was not detected by him, until he had reached middle age. •• ■■■/ •; ,; ■ ■/ . 'I Many people are unable to keep one eye shut and the other open at the same time. This is especially common with children, but less frequent with adults, and .it will be readily understood that in such cases considerable differences in the powers of the eyes may exist without being suspected. Sir John Herschel mentions the case of an elderly person who by chance made the unpleasant discovery that he was .altogether blind in one eye: Some simple easily performed experiments give interesting glimpses into the constitution of the eye. Go into a dark room with a candle. Looking' at the wall, which should be of a uniform dark tint, on moving the candle, up and down, close to the outer side of one eye, so that the light falls very obliquely into it, one of what are called " Purkinjo-'s figures " will be seen. This, is an appearance of a series of diverging and branched' red lines on a dark field, in the interspace of two or three of which is a sort of cup-shaped disc. These lines are the blood-vessels of the retina,, and the disc is the " yellow spot " or most sensitive part of the eye. ; In : the same situation, looking still at the dark wall and keeping the eye fixed on one point, hold the candle at arm's length and move it to and fro fora distance of about two inches on a level with the point towards which the eye is-' directed, and a little to the right or left of it, when a faint light may be seen moving in the opposite direction to the candle and on the other side pi- the point looked as. ..TJu'srJighfc, i£ ( more nearly ; inspected, is' seen to bean inverted image of the candle, flame, equal in size to it, but very faint, and is, in facts the light, of the candle reflected from the lens of ilie eye. Other internal parts of the eye also reflect some of the light that falls into it, but the images formed by them are not so easily seen. It is well known that the pupil contracts mechanically in bright light and expands in; faint light, but it has been found that, to some extent, this change is under the control of the will. A scientific observer, writing on tliis subject, says that when fronting a window or other light he can make the pupil expand or contract at the desire of anyone looking into his eye. The pupil is contracted while he steadily looks at the light, and when he wishes to expand it, all that is necessary is to take his? attention away from the eye and fix it on spnie other part of the body without moving his eyes. This he does, say, by biting his tongue, pinching his arm, or in some such way. The sensitiveness of the retina is diminished by this means, and the pupil, dilates. It contracts again when the mind is once' more recalled to the. eye and the bright light. Photographs .have been taken of the interior of the eye, some of which show the optic nerve and the bi'anching bloodvessels of the retina, and others show an inverted picture of the objects to which the eye was directed depicted on the retina. j Photographs of the eye in darkness, taken I by' means of the magnesium flash-light, show the pupil dilated to an extraordinary ! size, and the iris reduced to a mere ring. The fact has often been noted that at times, when a sudden idea or image pre- | sents itself to the; intellect, there is a ! distinct and sometimes even a painful sensation of luminosity produced in the eye, which is occasionally intense enough to be observable even in broad daylight. A current of electricity produces a sensation of light in the eye, and, of course, everyone has seen the " star 3 " which arise from a blow, on the head. . • ' Helmholtz has proved that the interior pf the eye is itself luminous, and he was able to see the movement of his arm, in total darkness, by the light of his own eye. Speaking of this in a recent lecture, Professor Tesla, the electrician, called it one of the most remarkable experiments recorded in the history of science, and said that probably only a . few men could satisfactorily repeat it, as very likely such luminosity of the oyo is only found in connection with uncommon activity of the brain and great imaginative power. There is a remarkable sympathy between the eyes. So much is this the case that any serious injury to the one is almost certain to affect the other, hence the necessity which often arises for, the removal of .tho.'.iivjured eye. mainly for the sake of saving the other. . This sympathy has been shown by Doctor Chauveau to extend bo far that colour perceived by one eye alone excites the retina of tho other. This is shown in the following way : If a white surface is observed for a short time through coloured glass with one eye, while the other eye is screened, on removing the coloured glass the white ground appears to be tinted with the colour complimentary to that of the glass. This is a common and well-known result, but the point is that if the first eye is closed and the screened eye opened, the whole surface appears tinted with the same colour as the glass. : There are many curious optical illusions connected with colour. If letters cut out of blue and red paper are stuck on a screen of black velvet and viewed from a suitable distance, the red letters appear to some people to be nearer than the blue, while about as many seethe blue nearer than the red. Looked at with one eye closed this difference disappears, and both sets of letters appear equally distant. If one who sees the led letters nearer than the blue covers his pupils on the outer or temporal side with a black screen the red will retreat, and soon appear' to be. behind the blue ; while if one who sees the blue as nearer than the red covers his pupils on the inner or' nasal side, the red appear to come forward and the blue to shrink back.

W Strange and Co. are now offering exceptional value in blankets and flannels.