188 SPECIAL PHYSIOLOGY 



(2) Make similar observations at 5.5 m. and 6 m. Each obser- 

 vation should be made three or four times and the average taken. 



(3) Record these averages in a table ruled with columns for the 

 values d, o, i and n. 



(4) Calculate for column n the values obtained by substituting, 



i d 



in the formula n= , the values observed in (1) and (2). What is 

 o 



the value of nf 



(5) Measure the anteroposterior diameter of the eye. How far 

 anterior to the posterior surface of the sclera is n located? How 

 far from the surface of the cornea? How does the ratio of these 

 two quantities differ from that given above for the human eye? 



(6) Is the image erect or inverted? Explain the phenomenon? 



(7) Move the eye to within 1 m. of the object. Note that a fairly 

 clear image may be thrown upon a posterior segment of the sphere, 

 which is many hundred times the area of the fovea centralis. 



(8) If a fine, sharp needle be thrust through the eyeball, following 

 a course perpendicular to the optical axis and cutting it at n, what 

 relation would this needle have with the lens? Would it be tangent 

 to the lens; would it enter the lens, or would it pass free of its pos- 

 terior surface? 



For these experiments the eye may be frozen after the introduction 

 of the needle and a vertical longitudinal section made. 



VI. ACCOMMODATION AND CONVERGENCE. 



In the above experiment with the excised rabbit's eye one notices 

 a marked blurring of the image when the eye is brought near the 

 object. Though the definition of the image is sharp at 5 m. to 6 m. 

 or beyond, at 2 m. or 3 m. the outlines are hazy. The normal living 

 eye is, however, able to give one the sensation of a clear image at any 

 distance from several inches to several miles. That there is actually a 

 sharply defined image upon the retina when the normal mind has 

 a sensation of such an image there is no doubt. 



One knows from his experience with optical instruments that they 

 must be readjusted for each distance if they are to yield a sharp 

 image for each distance. 



The same thing is true in the case of the organic optical instru- 

 ments with which one perceives the form, color, and space relations of 

 the objects of his environment. The functional adaptation of the 

 visual organs to distance is called accommodation. 



A. Accommodation. 



Experiments and Observations. (1) Take a sharp-pointed pen- 

 cil or similar object in each hand; hold the upturned points in the 



