VISION. 



the sclerotic coat free from the choroid, for about 3 mm. posterior 

 to the angle of the anterior chamber. Between the insertions of 

 the recti muscles, locate four points on the margin from which 

 incisions may be made antero-posteriorly. From these points, 

 make the incision posteriorly as far as the equator of the eyeball. 

 Dissect each flap free from the underlying choroid. After having 

 removed the pins fixing the recti muscles, draw the flaps back and 

 fix. Observe the iridal and ciliary portions of the choroid. 



(d) With a fine forceps, grasp the margin of the iris and with 

 small scissors cut out a sector with the ciliary body as a base. 

 Study the posterior chamber, suspensory ligament, and the anterior 

 surface of the ciliary body. 



(e) Make a circular incision with small scissors, severing choroid 

 and retina at about the line of the ora serrata. Lift off from the 

 vitreous humor the whole ciliary apparatus, placing it upon a 

 plate, anterior surface downward. Observe the posterior aspect 

 of the ciliary body. Describe the lens carefully, making a cross- 

 section. Can you discern its capsule? 



(I) Observe the retina, as seen through the vitreous, locating 

 the entrance of the optic nerve. Can you locate the f ovea centralis ? 



II. PHYSIOLOGICAL OPTICS. 



Light is propagated from a luminous point in every plane and 

 in every direction, in straight lines. These lines of direction are 

 called rays. Rays travel with the same rapidity so long as they 

 remain in the same medium; the denser the medium the slower the 

 passage of light through it. The divergence of the rays of light is 

 proportionate to the distance from which they come. 



Rays of light proceeding from infinity are parallel. In dealing 

 with rays of light which enter the eye, it will be sufficiently accurate 

 to consider them parallel when they proceed from a point more 

 than six metres distant. 



A ray of light, meeting with a body, may be absorbed, reflected, 

 or, if the body is transparent, refracted. In dealing with the eye, 

 it is necessary to consider only the latter. 



