MOTIONS OF THE EYEBALLS. 1 13 



violet surface, the result is no longer the same. To the retina dulled for red it 

 appears bluer. Similar observations may be made as regards the other 

 primary colours. 3. The image seen in the dark after the eye has been 

 directed to the sun is at first bright, then fades away and becomes red. If 

 while the red image is observed white light is admitted, it becomes green. 

 The excitation of the red elements is more persistent than of the others, and 

 their consequent exhaustion more prolonged. 



Colour-blindness. In some persons (in consequence, it 

 may be supposed, of defective excitability of the green or 

 red elements of the cones), red, in others green is mistaken 

 for grey. About two in every hundred railway officials 

 examined by Bonders, were found to present one or other 

 of these conditions. 



Motions (Rotations) of the Eyeballs. I. The straight line 

 which connects the apex of the cornea with the fovca 

 centralis retince is called the " visual axis." The plane in 

 which the visual axes of both eyes lie is the " visual plane." 

 When the visual axes of both eyes are directed to the 

 horizon and are parallel to the middle plane of the body, 

 the eyes are in the " position of rest " (primary position). 

 2. The vertical plane in which the visual axis lies when the 

 eyes are in the position of rest, is called the " vertical 

 meridian " and the horizontal plane at right angles to it, 

 the "horizontal meridian" 3. All rotations of the eye- 

 balls take place round axes (called " axes of rotation ") 

 which cut the visual axis at right angles, about 17 millim. 

 behind its mid-point. (Listing and Bonders.) 4. Any 

 position into which the eyeballs can be brought by rotat- 

 ing them from the position of rest directly upwards (i.e., 

 round horizontal and coincident axes of rotation), or directly 

 to the right or left (i.e., round vertical axes of rotation), is 

 called a secondary position. 5. In the position of rest 

 and in every secondary position, the horizontal meridians of 

 both eyes are in the visual plane. All other positions are 

 called " tertiary." In every tertiary position, the horizontal 

 meridians intersect the visual plane, at an angle which is 

 called the " angle of rotation." (See Exercises.) 



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