190 SPEC I A L PH YSIOL OGY 



other members of the class the minute details of the object, such as 

 slight irregularities of surface lines or other details. If an individual 

 is able to convince his comrades that he can perceive at this distance 

 the minute details of objects, he must be credited with normal vision. 

 Inasmuch as he can also see with the usual distinctness more distant 

 objects, the punctum remotum is said to be located at infinity; or, 

 to state it in another way, the eye is able, with suspended accommo- 

 dation, to bring parallel rays to a focus upon the retina. 



(6) It frequently happens that the individual under observation 

 fails to make out more than the merest outline of an object 6 m. 

 away. Decrease the distance until he is able to perceive details seen 

 by the majority of his comrades. If this distance has to be decreased 

 to 2 m. or 3 m. the determination may be made more exact by resort- 

 ing again to the needle and punctured card mentioned in (3), and 

 carrying the needle away until it appears double. 



In recording the punctum remotum, 1 write infinity ( oo) for 6 m. 

 or more, and for any distance within that record in metres and 

 decimals thereof. 



(5) How many metres from the punctum remotum to the punctum 

 proximum in those cases where the punctum remotum is less than 

 6 metres? 



(6) Observe the pupil closely while the subject directs the eye from 

 a distant object to a near one. It contracts slightly. One may assume 

 that this act of the iris is advantageous. Show from the standpoint 

 of theoretical optics why it is advantageous. 



(7) Observe from the side that when the act of accommodation 

 takes place the iris at the edge of the pupil not only moves toward the 

 center, but advances noticeably toward the cornea. What could 

 produce this? 



(a) If the edge of the iris rests upon the lens capsule, would it not 

 be pushed farther toward the cornea incident to its contraction toward 

 the center? 



If the pupil contracted from a 3 mm. diameter to a 2 mm. diameter, 

 how much would it advance incident to the normal curvature of the 

 lens (radius 10 cm.)? Could this be detected by the method of 

 observation which has been employed? 



(6) Account for the forward movement of the pupillary edge of the 

 iris during accommodation. 



B. Adaptation of the Eye for Direction. Convergence. 



Just as the eye possesses a mechanism by which it changes its- 

 refractive power for different distances, so it possesses a mechanism 



1 It must be stated here that this experiment does not make it certain that the punctum- 

 remotum is not beyond infinity. This would, however, be a pathological condition, and 

 need not be discussed here. There will be occasion to refer to this question more in detail 

 in a subsequent lesson. 



