186 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



Draw conclusions regarding the relative importance in accommodation 

 of these refractive surfaces. 



Repeat these observations with the help of the phacoscope (Helm- 

 holtz). 



Annotation. The phacoscope consists of a roughly triangular box. The ob- 

 server's eye is placed in the aperture at A and focalizes a far object through aperture 

 B. Orifice C contains two prisms, through which light is reflected upon the eye of 

 the observed person. The observer notes the images through the orifice D. The 

 observed person then accommodates for the pin situated in aperture B (Fig. 109). 



Since the image from the cornea does not change its position or shape, the 

 cornea does not alter its refractive power. The inverted image from the posterior 

 surface of the lens undergoes a very slight change, showing that this refracting 

 surface remains practically unchanged. Contrariwise, the image from the anterior 

 surface of the lens becomes more rounded and moves toward the corneal image. 

 This change proves that this refracting surface suffers the principal change in ac- 

 commodation. 



5. Wabbling of the Lens. Gaze upon a light wall or ceiling. Do 

 you notice black dots traversing the visual field (muscse volitantes)? 

 If you do, quickly accommodate for a near object and note that the 

 spots execute a jerky lateral movement. This fact indicates that ac- 

 commodation relieves the tension under which the lens is ordinarily 

 held and allows it to deviate somewhat from its optical axis. This 

 phenomenon constitutes the so-called wabbling of the lens. 



6. Formation of the Retinal Image. Accommodate for a light ob- 

 ject situated about 6 m. from the eye. Hold the index-finger of your 

 right hand in the visual axis of this eye. Draw a diagram to show 

 why the finger appears indistinct. Repeat this experiment, but accom- 

 modate for the finger. Draw a diagram showing why the far object 

 is not clear. 



Look at a window through wire netting held about 25 cm. in front 

 of the eyes. Later on accommodate for the netting. Explain. 



7. Scheiner's Experiment. Make two small holes in a card at a 

 distance of 4 mm. from one another. Close one eye and hold the open- 

 ings in front of the pupil of the other eye. Accommodate for a pin 

 held about 18 cm. in front of the eye. The pin is seen single. Now, 

 accommodate for a pin held at a distance of 60 cm. from the eye. The 

 far pin is seen single and the near pin double. Close the left opening 

 in the card and observe which image disappears when accommodating 

 for the far pin and which when accommodating for the near pin. Draw 

 diagrams showing the course of the rays, and explain the peculiar 

 blocking of the images following the closure of one or the other of the 

 openings in the card. 



The psychic element in vision is also clearly betrayed by the follow- 

 ing experiment: With the left hand hold a card bearing a pinhole about 

 3 or 4 cm. in front of the eye. The other eye should be closed at this 

 time. With the right hand bring the head of a pin from below into the 

 field of vision, adjusting the pin as close as possible to the pupil. Note 



