368 THE SPECIAL SENSES. 



condition is described as esophoria. A tendency to drift up or 

 down is called hyperphoria, and this is further specified as right 

 or left hyperphoria according to the eye whose axis deviates 

 upward. A lack of resting balance of this kind may make itself 

 felt also in near work, particularly in reading, sewing, etc., since 

 it will require a constantly greater innervation of the muscle 

 whose antagonist overbalances it. Under some conditions the 

 resulting muscular strain causes much uneasiness or distress. 

 The heterophorias are easily detected and measured by the use 

 of prisms, but they do not show the same constancy as the 

 refractive errors of the eye, owing probably to the fact that they 

 involve the variable factor of muscular tonus. 



The principle used in detecting heterophoria is to produce a condition of 

 diplopia by placing prisms in front of the eyes. If, for example, a prism (5°) 

 is placed in front of the right eye with its base up, the image of any object 

 looked at, a candle-flame, for instance, at a distance of 6 meters, will be dis- 

 placed on the retina above the fovea and will be projected, therefore, into the 

 lower half of the \dsual field. The left eye receives its image on the fovea and 

 two flames are seen one above the other, a condition of vertical diplopia. As 

 the images cannot be united by movements of the eyeballs the eyes come to 

 rest in their position of equihbrium. If the two flames lie in a vertical line the 

 eyes are in balance for distant vision. If the image belonging to the right 

 eye (in this case the lower image) is displaced toward the left hand as com- 

 pared with the other, exophoria is indicated. The pull of the external rectus 

 has thrown the front of the eye toward the right and its fovea in the oppo- 

 site direction, hence the image has fallen to the right of the fovea and is dis- 

 placed in the visual field toward the left. A deviation in the other direction 

 would indicate esophoria, and the amount in each case is expressed by the degree 

 of prism necessary to bring the objects into a vertical line (for esophoria the 

 correcting prism is placed base out, for exophoria, base in). In a similar way 

 hyperphoria is detected by producing in the beginning a condition of lateral 

 diplopia, by placing a suitable prism before the eye with its base in. Several 

 systems of notation have been proposed for prisms. The one commonly used 

 expresses in degrees the angle (refracting angle) at the edge of the prism. 



The defect may be remedied by surgical operations upon the 

 muscles, or by the use of proper prisms with their bases so adjusted 

 as to help the weaker muscle. In exophoria, for example, the 

 greater pull of the external rectus rotates the front of the eye 

 outward, while the back of the eye with the fovea is moved inward 

 toward the nose. A prism of the proper strength placed before 

 the eye with its base in toward the nose will throw the image of 

 an external object on the fovea, without necessitating a con- 

 traction of the internal rectus to bring the fovea back into its nor- 

 mal position. When the lack of balance between the opposing 

 muscles is so great that the visual axes cannot by muscular 

 effort be brought to bear upon the same points, we have the condi- 

 tion of squint or strabismus. Such a condition may result from 

 a deficiency in strength or in actual paralysis of one or more of the 



