THE EYE AND VISION. 119 



II. ASTIGMATISM. 



Test your own eyes for astigmatism with a card having 

 lines of the same brilliancy radiating from the centre, or 

 with the face of the clock. Astigmatism is present if 

 some of the lines are more distinct or brighter than 

 others. 



Astigmatism is usually caused by unequal curvatures 

 of the cornea. 



F. Changes in the Iris and the Pupil. 



Material. A living cat. 



Apparatus. A gas-jet or lamp. A hand-mirror. A piece of 

 cardboard about six inches square. 



I. CHANGES IN THE SIZE OF THE PUPIL. 



Sit opposite a light and study the pupils of the eyes 

 in a hand-mirror. Place a hand over one eye and notice 

 the diameter of the pupil of the other. Suddenly remove 

 the hand from the eye while watching the pupil of the 

 open eye. What change do you observe? 



II. THE RATE OF MOVEMENT OF THE IRIS. 

 a. The human eye. 



Close the left eye, and with the right look through a 

 pin-hole in a card at a white shade of a lighted lamp. 

 Notice the size of the circular visual field, or the amount 

 of space visible at one time. Is it very bright? Open 

 the left eye. What change is there in the visual field? 

 Again close the left eye and note the change in the shape 

 and brilliancy of the visual field. 



