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THE POPULAR SCIENCE MONTHLY. 



called the yellow spot of Sommering, 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 suc- 

 cessively upon the yellow spot, and, that you may do so readily, the 

 words are arranged in straight, horizontal lines. The eyeball, other- 

 wise 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 com- 

 bining 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 apparent- 

 ly not so important in observation as those just described. 



Fig. 1. 



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 con- 

 tracting more rapidly than the other. A curve is therefore more diffi- 

 cult to observe, or run the eye over, than a straight line, and the diffi- 



