236 PEACTICAL PHYSIOLOGY 



EXPERIMENT IT. Make the subject close one eye and shade the 

 open eye from the direct light. Observe the size of the pupil when the 

 eye is shaded. Then remove the shade ; the pupil will be seen to 

 diminish in size. From this experiment it may be inferred that the 

 amount of light entering the eye is controlled by the iris. 



[ADVANCED EXPERIMENT. Make a pinhole near the edge of a card, 

 and hold the card about 15 centimetres from the right eye, so that it does 

 not interfere with the field of the light. Let a good source of light be 

 placed about 60 centimetres from the eye, and allow a thin paper-screen 

 to shield the light from the right eye. The left eye, when open, will look 

 directly at the light, the right eye at the pinhole, and the illuminated 

 paper through the hole. Close the left eye, and accommodate as 

 nearly as possible for the distance of the pinhole. Note the size of the 

 hole. Then alter the accommodation by attempting to look far away 

 through the pinhole. The hole will immediately become distinctly 

 larger, though less definite, on account of the blurring of the edges. 

 Keep varying the accommodation, and the edge of the hole will 

 similarly vary. 



Whilst accommodated for far distance open the left eye. The 

 sudden entry of light in the left eye will cause reflexly a diminution in 

 size of both pupils. The pinhole will now become smaller. Close the 

 left eye again and it enlarges. The size of the blurred image of the 

 pinhole depends upon the size of the pupil, and hence variations in size 

 of the pupil appear as variations in size of the pinhole.] 



5. The changes in the Lens during Accommodation. Purkinje Sanson 

 Images. During accommodation for a near object, the ciliary muscle 

 contracts, with the consequence that the suspensory ligament is 

 slackened. The lens by its natural elasticity becomes more curved 

 in its anterior aspect, and its thickness through the optical axis is 

 increased. This change of curvature can be measured by means of the 

 ophthalmometer. The existence of such a change may be inferred 

 from the following experiments in which observations are made upon 

 the images reflected from the anterior surface of the cornea, the 

 anterior surface of the lens, and the posterior surface of the lens. 



EXPERIMENT I. (PRELIMINARY). In a dark room place on a table, 

 rather to the right of the observer, a convex lens mounted on a stand. 

 Hold a watch glass a few inches in front of the lens, with the convex 

 surface of the glass forward. Still more to the right let a lighted 

 candle be placed. The candle and the observer's eye should be 

 symmetrically arranged on either side of the optic axis of the lens and 

 watch glass. Observe the images reflected from the surface of (a) 

 the watch glass ; (b) the anterior surface of lens ; (c) the posterior 



