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what was wanting on the side of precision, they might arrive, after the lapse of ages, at a sufficiently exact knowledge of the measure of their country. Moreover, they knew how to draw a meridian line, as the placing of the faces of the great pyramid to the four cardinal points, sufficiently proves. They were therefore competent to take the azimuth of a triangle. Lastly, they knew how to make use of the gnomon; and after their taste for every thing colossal, there is no doubt but that they erected some very large ones, which might have given the proportion. of the shadows with so much the more exactness, as it was possible to multiply and combine observations. Nothing more is wanting, to find with all the precision that the Egyptians have attained to; 1st, the difference in latitude of two points; 2dly. their distance expressed in the measure of the country.

ART. VI. Experiments and Observations relative to Vision ; by Marshall Hall, M. D. of Nottingham, formerly Senior President of the Royal Medical Society of Edinburgh.

THE following detail is nearly confined to a series of obser

vations and experiments made by myself. I have been induced to adopt this plan partly from the difficulty of meeting with persons sufficiently interested in their results to prosecute experiments at once nice and difficult in themselves, and requiring a certain degree of the power and habit of abstraction, for their performance; but principally from a peculiarity in my own vision, by which I am enabled to give the subject of this paper a peculiar illustration.

The peculiarity of vision to which I allude, consists in an ability to adapt the left eye for distinct vision at shorter distances than the right, and in an incapacity for adapting the left eye for distinct vision at great distances, whilst the right eye possesses the power of adaptation for distinct vision at very considerable distances. The nearest distance at which a

bright point is distinctly seen by the right eye is 4 inches; but by the left eye the point is seen with perfect distinctness at the distance of 3 inches. The same point is seen distinctly by the right eye at the distance of 17 inches; by the left it is seen indistinctly at any distance beyond 14 inches. With the right eye I distinguish each small branch and each leaf on a tree planted about thirty yards from my window; with the left eye these objects are seen in the most indistinct and confused manner. A distant light seen distinctly, or as a point nearly by the right eye, appears magnified into a large star to the left. The distant object seen thus indistinctly by the left eye, immediately acquires distinctness by the use of

a concave lens.

A number of experiments have convinced me that, in myself at least, ordinary vision is performed principally by one eye alone, the left eye being chiefly employed and adapted for distinct vision at short, and the right eye, at long distances; whilst the axis of the other eye is merely directed to the object, in order to prevent the confusion and double vision which would arise from the different direction of the two eyes. In proof of this observation I may observe, that when the eyes are directed to a distant object, as the tree before my window just mentioned, any intervening object, placed within certain limits with respect to distance, and seen of course double, appears indistinct to the right eye, but perfectly distinct and with a well defined outline, to the left. On the contrary, when the characters on a printed page placed at the distance of about eight inches from the eye, are observed, whilst the point of a pen-knife placed at the distance of six inches is seen single and distinctly by both eyes, each word and line is of course seen double, and the right part of the double image, or that seen by the right eye, appears distinct, whilst the left side of this image, or that arising from vision by the left eye, is seen indistinctly and obscurely.

When the eyes are fixed on a distant object, and an intervening object placed also at a considerable, although at less distance, is observed, it is seen nearly distinctly by the right eye, and less distinctly by the left; and there is a particular

intervening distance at which it is seen equally indistinctly by both eyes. The same remark applies, mutatis mutandis, to the experiment in which the characters of a printed page are placed at the distance of eight inches from the eyes, and observed whilst the eyes are fixed for single and distinct vision at a shorter distance; if the less distance be nearly eight inches, the characters of the printed page placed beyond it, are seen either almost distinctly by the left eye, or equally indistinctly by both eyes.

These remarks appear to show that, in myself, as the right eye is endowed with a longer, and the left with a shorter sight, so in observing near or distant objects, the left or the right eye is principally employed, and most adapted for distinct vision, whilst the axis of the other eye is directed to the object, in order to obviate the double vision which would take place were this axis allowed to take any other direction; I may therefore be said to look at the object with both eyes, but to examine it with one only.

Having made this statement respecting the condition of my own eyes, and of their different capacity for adapting themselves for distinct vision at different distances, I now proceed to the detail of some experiments and of some cases of vision, in which the object is not only seen indistinctly and with an undefined margin, but also fringed with the prismatic colours.

In the first place I may observe, that in the cases of indistinct vision already described, if the object be opaque and well defined in itself, its borders are manifestly tinged by a decomposition of the rays of light. This fact observed in general in the observations and experiments already described, is evinced still more distinctly in the following manner.

If both eyes are fixed, adapted for single and distant vision, at the distance of eight feet, and if an intervening object within the distance of about six inches, or beyond that of about twenty inches, be glanced at, the latter object is seen indistinctly and bordered with the prismatic colours, by both eyes, but in a different degree and in a different manner by each. But if, in this experiment, the intervening object be

placed beyond the distance of six inches and within that of twenty, the right side of the double image, or that seen by the left eye, is distinct and free from colours; whilst the image induced by the impression of the light on the right eye, appears indistinct and fringed as before. By three other gentlemen, the more general observation has been made that, whilst the eyes remain adapted for the single and distinct vision of a more distant object, a nearer one, a word in capitals on a neatly printed page for instance, is observed to be bordered by the prismatic colours from a decomposition of the rays of light.

In these experiments a straight line on a printed page becomes doubled, presenting the appearance of two light blue lines inclosing a line or space of a brightish yellow colour; a dot becomes a small circle of light blue having a centre of yellow; an o becomes three concentric rings,-of blue, yellow, and blue; and if two o's be viewed nearly together, as in the word GOOD, the light blue borders are seen to coalesce at the parts which approach each other, in the manner of two penumbræ, and to give origin to an appearance of a deeper blue.

These experiments have been diversified in the following manner. The eyes have been fixed on an object placed near them, so as to see it singly and distinctly. They have then been glanced towards another object, such as a word on a printed page, placed at a greater distance. The latter object is of course seen double; the right side of the double image, now induced by an impression made on the right eye, is distinct and free from colours; the left is indistinct and fringed with the prismatic rays. The three gentlemen before alluded to, observed the appearance of coloured fringes in general, whenever the eyes were adapted for distinct vision at a near distance, and glanced at a printed page or other proper object placed somewhat beyond the former.

Having thus ascertained that, when the eyes were glanced at a well defined object situated at a different distance from that at which they are at the moment adapted for distinct vision, whether greater or less, the rays of light are decomposed

in their passage to the retina; a set of experiments were next made by properly placing concave or convex lenses, or plane, convex, or concave mirrors, with respect to the eyes and the object viewed, so as to vary the degree of divergency of the rays proceeding from them. When the object is seen distinctly by the eye alone, it is seen indistinctly and fringed by the decomposition of the rays of light, when viewed by means of any of these instruments, the conformation of the eye remaining unchanged: and vice versa, when seen distinctly by means of any one of these instruments, it appears indistinct and coloured without them.

I have observed that in myself there is a certain distance with regard to each eye, and different for each, being greater for the right than for the left, at which an object cannot be made to appear fringed with colours by attempting to fix the vision at a point beyond it. Beyond this distance the margins of a small object cannot be seen distinctly by either eye; by the left they are seen indistinctly and slightly fringed with colours; with regard to the right, the distance alluded to is too great to allow of a small object being examined with sufficient minuteness.-There is in the same manner, a certain short distance with respect to each eye, and less for the left than for the right, at which an object cannot be made to display the fringe of prismatic colours by endeavouring to fix vision at a still smaller distance.-The distances just alluded to, are the limits of distinct vision for each eye respectively.

It would appear from this view of the subject, that whenever the eye is glanced at an object situated at a distance different from that at which the eyes are adapted for distinct vision, it produces the appearance of prismatic colours by decomposing the rays of light and leaving this dispersion without connection. In distinct vision, on the contrary, the decomposition of light appears to be accurately connected, so as to leave no appearance of prismatic colours. In distinct vision the eye appears to be perfectly achromatic; in the cases of indistinct vision which have been described, it appears to have lost its achromacy. What is the rationale of this phenomenon? Before

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