184 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



2. Formation of the Image Upon the Retina. Place a fresh ox eye 

 in a watch-glass. Make a small square opening in its upper wall directly 

 behind the ciliary body. Direct the pupil of this eye toward an incan- 

 descent lamp (optical lantern), the rays of which have been rendered 

 parallel by the interposition of projection lenses. Vary the distance 

 between the light and the eye until the rays have been made to inter- 

 sect sharply upon the retina. Place a diaphragm with an upright arrow 

 in front of the projection lens. Note the condition of the image. By 

 means of a diagram show why the image must be inverted. Move the 

 incandescent lamp (optical lantern) slightly in different directions and 

 observe the direction of the movement of the image. 



Endeavor to thin the eyeball in the region of its posterior pole by 

 removing the sclera. This means is frequently resorted to in order to 

 obtain a clearer image. It illustrates the manner of focalizing an 

 object upon the ground glass of a photographic camera. 



3. Changes in the Size of the Pupil. Light Reflex. Observe the 

 changes in the size of the pupil in a subject who alternately accom- 

 modates for a window and a darkened wall. Cover the eye of the sub- 

 ject with the flat of your hand for a few moments. Remove the hand. 

 Note the constriction of the pupil. Again close one eye with your 

 hand and observe the size of the pupil of the other eye. Suddenly re- 

 move the hand. Note that the pupil of the other eye also constricts. 



The cornea acts as a planoconvex glass and gives a larger size to the 

 pupil than it actually possesses. To show this take an ox eye, the 

 cornea of which has been removed. Place a watch-glass in front of it. 

 The pupil immediately appears larger. 



In order to show the movements of the iris in your own eye proceed 

 as follows : With your right eye look at a uniform white surface through 

 a pinhole in a card, preferably at the white shade of a reading lamp. 

 Close your left eye. Obtain a concept of the size of the circular visual 

 field. Open the left eye. The field becomes smaller and brighter, 

 owing to the constriction of the pupil. Again close the left eye. The 

 field gradually enlarges and is slightly dulled, owing to the dilatation 

 of the pupil. 



Accommodation Reflex. Observe the changes in the size of the pupil 

 in a subject who alternately looks at objects near to and far away from 

 his eye. 



4. Changes in the Shape of the Lens. Look at the eye of the sub- 

 ject from the side, observing the position of the iris when accommodated 

 for a far object. Ask the subject to accommodate for a near object. 

 Note that the iris is forced forward into the aqueous chamber, owing to 

 the fact that the lens now becomes more convex. 



Insert, an ordinary watch-glass in a tube of black paper, its con- 

 vexity being turned outward. A few centimeters behind it adjust a 

 biconvex lens. Hold the tube toward a candle, and note the three 

 images reflected from this system, viz., one from the watch-glass (cor- 

 nea), one from the anterior surface of the lens, and one from the pos- 



