PRACTICAL EXERCISES 987 



looks through the hole B ; the observed eye is placed at a hole oppo- 

 site the hole A. The candle or the observed eye is moved till the 

 observer sees three pairs of images, one pair, the brightest of all, 

 reflected from the anterior surface of the cornea ; another, the 

 largest of the three, but dim, reflected from the anterior surface of 

 the lens ; and a third pair, the smallest of all, reflected from the 

 posterior surface of the lens (Fig. 387, p. 906) . 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 ; therefore 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 under- 

 goes a change ; (b) that 

 the change is increase of 

 curvature (diminution of 

 the radius of curvature), 

 for the virtual image re- 

 flected from a convex 

 mirror is smaller the 

 smaller is its radius of 

 curvature. (The third pair 

 of images really undergo FIG. 434. PHAKOSCOPE. 



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 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 accommodated 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. 435). To determine the near-point of distinct vision (p. 915) 

 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 in- 

 serted vertically into a cork, along the rule, beginning at the end 

 farthest from the eye, until with the strongest effort of accommoda- 



