VISION. 215 



point where the ideal refracting surface of the eye 

 cuts the optical axis, by applying the formula: 



p=/m. 



Assuming for /JL the value which it has been calculated 

 to have in the human eye (1.3365 Landolt, p. 86), 

 how far is this point posterior to the anterior surface 

 of the cornea ? How does your result compare with 

 that for the " reduced human eye? " 



(7) Is the image erect or inverted? Explain the phe- 

 nomenon ? 



(8) Move the eye to within one meter of the object. 

 Note that a fairly clear image may be thrown upon a 

 posterior segment of the sphere, which is many hun- 

 dred times the area of the fovea centralis. 



(9) 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? 



(10) If a similar experiment were performed with refer- 

 ence to the point p, what relation would the needle 

 have to the anterior surface of the lens ? 



For these experiments the eye may be frozen after 

 the introduction of the needle and a vertical longi- 

 tudinal section made. 



