PRACTICAL EXERCISES 1103 



p. 1021). The last two pairs can, of course, only be seen within the pupil. 

 The observed eye is now focussed first for a distant object (it is enough 

 that the person should simply leave his eye at rest, or imagine he is 

 looking far away), and then for a near object (an ivory pin at A). 

 During accommodation for a near object no change takes place in the 

 size, brightness, or position of the first or third pair of images; there- 

 fore the cornea and the posterior surface of the lens are not altered. 

 The middle images become smaller, somewhat brighter, approach each 

 other, and also come nearer to the corneal images. This proves (a) that 

 the anterior surface of the lens undergoes a change ; (6) that the 

 change is increase of curvature (diminution of the radius of curvature), 

 for the virtual image reflected from a convex mirror is smaller the 

 smaller is its radius of curvature. (The third pair of images really 



undergo a slight change, such 

 as would be caused by a small 

 increase in the curvature of 

 the posterior surface of the 

 lens ; but the student need 

 not attempt to make this 

 out.) 



4. Schemer's Experiment. 

 Two small holes are pricked 

 with a needle in a card, the 

 distance between them being 

 less than the diameter of the 

 Fig. 474. Phakoscope. pupil. The card is nailed on 



a wooden holder, and a needle 



stuck into a piece of wood is looked at with one eye through the holes. 

 When the eye is accommodated for the needle, it appears single; when 

 it is accommodated for a more distant object, or not accommodated at 

 all, the needle appears double. The two images approach each other 

 when the needle is moved away from the eye, and separate out from 

 each other when it is moved towards the eye. When the eye is ac- 

 commodated for a point nearer than the needle, the image is also 

 double; the images approach each other when the needle is brought 

 closer to the eye, and move away from each other when it is moved 

 away from the eye. If while the needle is in focus one of the holes be 

 stopped by the finger, the image is not affected. When the eye is 

 focussed for a greater distance than that of the needle, stopping one 

 of the holes causes the image on the other side of the field of vision 

 to disappear; if the eye is focussed for a smaller distance, the image 

 on the same side as the blocked hole disappears (Fig. 475). To de- 

 termine the near-point of distinct vision (p. 1029) the card may be 

 mounted vertically on a cork, and this fastened by a rubber band to 

 the end of a foot-rule. Move a needle, also inserted vertically into a 

 cork, along the rule, beginning at the end farthest from the eye, until 

 with the strongest effort of accommodation it is seen double. Then 

 push it back slightly to the point at which, again with maximum 

 accommodation, it is just seen single. Repeat the measurement with 

 a needle mounted horizontally. If regular astigmatism is present, 

 the distances will not be the same. Most eyes have slight regular 

 astigmatism. 



In myopic persons the far-point of distinct vision can also be de- 

 termined by Schemer's experiment. The needle being left on a shelf 

 at the level of the eye, the person walks away from it backwards, re- 

 garding it all the time through the perforated card, till it is no longer 

 seen single. 



