188 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



9. Circles of Dispersion. Move the screen farther forward and then 

 farther backward. Note that the rays now strike the retina widely 

 apart, i. e., they form a dispersion circle upon this receptor. 



The same end may be attained by moving the lantern either farther 

 away or nearer to the optical box (lens). Draw diagrams showing the 

 course of the rays under these two conditions. 



10. Near Point. Again adjust the screen (retina) in such a way that 

 the rays of light emitted by object L are brought to a sharp intersecting 

 point (emmetropia) . With a ruler determine the distance between 

 object L and the lens of the optical box. This distance constitutes the 

 anterior focal distance of this lens and indicates the position of its near 

 point. 



Move the lantern closer to the optical box. Observe that the rays 

 immediately pass into dispersion and finally leave the posterior surface 

 of the lens very divergently, illuminating the screen widely. A precise 

 focal point cannot be obtained if the object is situated inside the near 

 point. 



11. The Ametropic Eye. Hypermetropia. Myopia. Construct an 

 eye that is too short. To accomplish this end move the screen 2.5 cm. 

 in front of the position occupied by it when the eye was emmetropic. 

 This adjustment simulates the condition of hypermetropia or far-sighted- 

 ness. Theoretically speaking, the focal point of these rays lies behind 

 the screen. In order to render this eye emmetropic place a convex 

 lens in front of the aperture in the box (cornea), its strength being just 

 sufficient to converge the rays so that they intersect upon the screen. 

 A lens of 2 D. should accomplish this end. If a number of different 

 lenses are available, other degrees of hypermetropia may be established. 



Construct an eye that is too long. To accomplish this end move the 

 screen a distance of 2.5 cm. behind the position occupied by it in the 

 emmetropic eye. Note that the intersecting point of the rays now lies 

 in front of the screen (vitreous humor) and that the rays then diverge 

 and strike the screen in dispersion. This condition is known as myopia 

 or near-sightedness. In order to correct this condition interpose a 

 concave lens in front of the optical box (cornea). The entering rays 

 are thereby rendered more divergent, so that they are focalized exactly 

 upon the retina. A 2 lens ( 2D.) will accomplish this end, provided 

 the degree of myopia which has been established is not greater than 

 specified. 



Draw diagrams to show the manner of refraction in hypermetropic 

 and myopic eyes, and also indicate how these errors may be corrected. 



At the close of this exercise replace every lens in its proper paper 

 envelope. 



