THE SENSE OF SIGHT. 



673 



(medial torsion); the inferior rectus moves the cornea downward, slightly 

 inward, and twists the upper part away from the nose (lateral torsion); 

 the superior oblique displaces the cornea downward, slightly outward, and 

 produces medial torsion; while the inferior oblique moves the cornea upward, 

 slightly outward, and produces lateral torsion. These facts show that for 

 certain movements of the eye at least three muscles are necessary (see 

 following table): 



Inward and 

 downward. 



Outward and 

 ui 



Outward and 

 downward . 



Rectus internus. 

 Rectus inferior. 

 Obliquus superior. 

 Rectus externus. 

 Rectus superior. 

 Obliquus inferior. 

 Rectus externus. 

 Rectus inferior. 

 Obliquus superior. 



If both eyes have their line of vision in the horizontal plane parallel 

 with each other and with the median plane of the body, they are said to be 

 in the primary position. All other positions are called secondary and tertiary. 

 Both eyes always move simultaneously, which is called the associated move- 

 ment oj the eyes. There are three forms of associated movements: (i) move- 

 ment of both eyes in the same direction; (2) movements of convergence by 

 which the visual lines are converged on a point in the middle line of the body; 

 (3) movements of divergence, by which the eyes are brought back from 

 convergence to parallelism, or even to divergence, as in certain stereoscopic 

 exercises. A combination of (i) and (2) or of (i) and (3) takes place for 

 certain positions of the object looked at. 



Color-perception. A beam of sunlight passed through a glass prism 

 is decomposed into a series of colors red, orange, yellow, green, blue, and 

 violet the so-called spectral colors, so well exemplified in the rainbow. 

 The spectral colors are termed simple colors, because they cannot be any 

 further decomposed by a prism. Objectively, the spectral colors consist of 

 very rapid transverse electro-magnetic vibrations of the ether, from about 

 400 millions of millions per second for red to about 760 millions of millions 

 for violet, but subjectively they are sensations caused by the impact of the 

 ether-waves on the percipient layer of the retina. 



It is possible to mix or blend these spectral color-sensations in the eye 

 by stimulating the same area of the retina by different spectral colors, either 

 at the same time or in rapid succession. The following table shows the 

 results of such experiments as performed by v. Helmholtz (Dk. =dark; 

 Wh.=whitish): 



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