BINOCULAK VISION 869 



corresponds to the power which is required to correct the divergence 

 of the rays, i.e., the hypermetropia. Thus, if a lens of 5D is employed 

 and the movement of the light remains with the rotation of the mirror 

 until a little within a distance of 1 m. but is reversed at a distance of a 

 little more than 1 m., the point of reversal is at 1 m. Consequently, 

 ID of the strength of this lens is required to converge the rays, 

 while 4D of the total 5D have been made use of in overcoming the 

 divergency of the rays upon their projection from the observed eye. 

 In this case, the hypermetropia equals 4D. 



In astigmatism the same method may be followed, but these 

 tests must then be repeated for different meridians, i.e., the point 

 of reversal must be ascertained separately for the horizontal, vertical 

 and oblique planes. 



CHAPTER LXXIV 

 BINOCULAR VISION 



The Movements of the Eyeballs. — The organ of vision consists 

 of the globe of the eye, measuring nearly an inch from side to side, 

 slightly less than an inch from above downward and somewhat 

 more than an inch from before backward. Its volume equals 6.5 

 cm. and its weight nearly 7 grams. Connected with it externally 

 are different muscles, nerves and blood-vessels. It is supported by a 

 quantity of fat and connective tissue, the latter forming a lymphatic 

 space known as the capsule of Tenon. Within this capsule the eyeball 

 is made to move by the contraction of a set of muscles, designated as the 

 ocular muscles. These are the four recti and two oblique muscles. 

 The former, which are known respectively as the superior, inferior, 

 external and internal, take their origin from a tendinous ring investing 

 the optic foramen and sphenoidal fissure. From here they pass 

 forward along the walls of the orbital cavity and finally perforate 

 the aforesaid lymphatic space to gain access to the equatorial region 

 of the eyeball. Closely investing the latter, they finally terminate 

 in their respective positions about 7 mm. posterior to the margin of 

 the cornea. The superior oblique muscle arises from a small tendon 

 upon the inner margin of the optic foramen and, passing forward to 

 the inner angle of the orbit, terminates in a rounded tendon which 

 plays in a pulley of fibro-cartilaginous tissue suspended from the 

 depression in the internal angular process of the frontal bone. From 

 here this tendon is reflected backward, outward and downward upon 

 the outer part of the eyeball about midway between its cornea and the 

 entrance of the optic nerve. The inferior oblique muscle arises from a 

 depression in the orbital plate of the superior maxillary bone, external 



