VISION 117 



opposite the needle. B looks through the hole at the 

 side. A lamp or candle is placed at some little distance 

 from the prisms and shifted about until B sees on the 

 eye of A, when the latter looks at a distant object, two 

 small bright patches of light on the cornea, two larger 

 but dimmer patches on the anterior surface of the lens, 

 and two small and very dim (not readily seen) patches 

 on the posterior surface of the lens. 



Let A now accommodate for the needle in front of 

 him, making every effort not to move the eyeball. B 

 will see the two patches on the anterior surface of the 

 lens approach each other, while the other two pair re- 

 main motionless, thus showing that during accommoda- 

 tion for near objects the anterior surface of the lens be- 

 comes more convex. 



Observe that in accommodating for near objects the 

 pupil becomes smaller, and in accommodating for far 

 objects the pupil becomes larger. 



8. Astigmatism, a. Draw on a card a star composed 

 say of eight lines passing through the centre, the angle 

 between each' two neighbouring lines being the same 

 and the lines of equal tint and of equal thickness. 



Place this at about the distance which has been deter- 

 mined (4) as the far limit of accommodation (if this 

 distance is more than eight or ten yards use convex 

 spectacles.) 



Probably one or more of the lines will be seen much 

 more distinctly and with less blurring than the others. 

 Approach gradually nearer the star, and note whether the 

 other lines become all visible at once or in succession. 



Repeat this first with one and then with the other eye 

 closed; the astigmatism may be different in the two eyes. 



