THE SENSE OF VISION 



631 



iact that in the passive position the eyeballs coincide with the axes 

 -of the eye-sockets, which are somewhat divergent. 



Normally, double vision, or diplopia, is prevented by the balanced 

 action of the ocular muscles ; when these muscles are relaxed, except 

 for their normal tone, the visual axes of the two eyes are parallel. 

 It sometimes happens that the balance is not perfect, and that 

 -a constant action of one or more muscles is necessary to keep 

 the visual axes parallel for distant vision a condition known as 

 heterophoria. If in the relaxed condition of the eye muscles the eye 



FIG. 358. To ILLUSTRATE TIIK HOROPTER. 



A is the point of regard; its retinal images are formed on the yellow spots at a, a'. 

 b, b' are corresponding points of the two retinae, therefore the distance a 6==the 

 distance a' b'. n, n' are the nodal points of the two eyes.- The angles a n b, 

 a' n' b', are equal to each other and to the opposite angles, A n B, A n' B ; and in 

 the triangles B C n, A C n', the opposite angles at C are equal; therefore, in 

 these triangles the remaining angles at A and B are equal. Therefore the point 

 B by which the corresponding images b, b' are formed must lie in the arc of a 

 circle passing through the points B. n, and n'. Similarly, it may be shown that 

 any other point forming corresponding images on the two retina) will lie in the 

 circular arc n A n'. 



turns outwards, the condition is known as exophoria; if inwards, 

 esophoria; if upwards or downwards, hyperphoria. Such a condition 

 throws a strain upon the eye muscles, and entails considerable ocular 

 distress. The trouble is corrected by the prescription of prisms, 

 or by operation. When the defect is so marked that it cannot be 

 corrected by muscular action, we have the condition of strabismus, 

 or squint. This results either from the overaction of one or more 

 of the muscles, or from a deficient action or paralysis of one or more 

 muscles. In the former case, it may be cured by surgical operation. 



