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A MANUAL OF PHYSIOLOGY 



a third pair, the smallest of all, reflected from the posterior surface 

 of the lens (Fig. 292). The last two pairs can, of course, only be 

 seen within the pupil. The observed eye is now focussed first for * 

 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 an- 

 terior surface of the lens undergoes a change ; (b) 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 



FIG. 331. SCHEINER'S EXPERIMENT. 



In the upper figure the eye is focussed for a point farther away than the needle ; in 

 the lower, for a nearer point. The continuous lines represent rays from the needle, 

 the interrupted lines rays from the point in focus. But the lines inside the eye, which 

 by an error in engraving are drawn as continuous lines, ought to be interrupted, and 

 vice -versa. 



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.) 



3. 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 



