XXVIL] VISION. 279 



A lamp or candle is placed at some little distance 

 from the prisms and shifted about until the 

 observer sees on the eye of the observed, 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 the observed now accommodate for the 

 needle in front of him, making every effort not 

 to move the eyeball. The observer will see the 

 two patches on the anterior surface of the lens 

 approach each other, while the other two pair 

 remain motionless, thus shewing that during 

 accommodation for near objects the anterior 

 surface of the lens becomes more convex. 

 Observe that in accommodating for near objects 

 the pupil becomes smaller, and in accommo- 

 dating for far objects the pupil becomes larger. 



7. Astigmatism. 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 

 determined ( 5) as the far limit of accommoda- 

 tion (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 



