called the yellow spot of Sömmering, and to this distinct vision is limited, for we see clearly only the part of an image that falls within it. It is even doubtful whether we see at one time distinctly, or, in other words, can observe, more than a point in that image. If you look at the middle of this page, you really see clearly only the point directly before your eye. The rest is indistinct, and, to observe a word on another part of the page, you must move the eye so that its image may fall on the yellow spot. So in reading, you run the eye over the words, or, by moving the eye, cause their images to fall successively upon the yellow spot, and, that you may do so readily, the words are arranged in straight, horizontal lines. The eyeball, otherwise immovable, may be rotated in its socket by the action of muscles, of which, in each eye, there are four principal ones, arranged in pairs, as in Fig. 1. When A contracts, the pupil is turned in the direction B A. The pair B A then cause the eye to rotate from side to side, while the pair C D cause it to rotate in a vertical plane. By combining two contiguous muscles, as, for instance, A and C, we may move the eye obliquely in any direction. Of the oblique muscles represented in the diagram I will not here speak, as they are apparently not so important in observation as those just described.
| Fig. 1. | Fig. 2. |
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If I look at the middle of a straight, horizontal line, my head being held erect, the image of that line (a b, Fig. 2) will lie on the retina directly between the muscles A B, the central point falling in the middle of the yellow spot S. In running my eye over that line, I use the muscles A B in such a way as to draw the image through the yellow spot; and, if, in doing so, I use these muscles with perfect regularity, I say the line is straight. Perpendicular and horizontal straight lines are the more easy to examine, because their images fall directly between two opposing muscles. An oblique line is difficult to examine, and we instinctively turn the head, in order to bring it in the plane of rotation of one or the other set of muscles. In following a curved line with the eye, two muscles are used together, one contracting more rapidly than the other. A curve is therefore more difficult to observe, or run the eye over, than a straight line, and the diffi-
