AX EXPERIMENTAL PROOF OF INVERTED 

 RETINAL IMAGES* 



By a. H. Patterson 



It is usually somewhat mystifying to be told that all up- 

 right objects, such as trees, men walking, etc., form inverted 

 or upside-down images on the retina of the eye. However, 

 it is easy to construct a simple bit of apparatus which will 

 prove the point in question. But first we must understand 

 clearly one or two principles of the action of light rays. 



Take a sheet of cardboard S and pierce in it a small hole 

 about one-tenth of an inch in diameter. In front of it place 

 a lighted candle, and behind it a cardboard screen S\ On 

 this latter screen will be seen an inverted image of the candle 

 — a so-called "pin-hole image." Now j)lace the candle quite 

 close to the screen 8 and let its light shine through the small 

 hole upon the screen S\ Then take a third cardboard screen 

 S'\ from the middle of which is cut a hole 1 inch in 

 diameter, and place it between the two screens ^S and S\ 

 The divergent pencil of rays coming through the hole 

 in ^S* and the inch hole in S" will illuminate a 

 circular area on the screen S' slightly more than an inch in 

 diameter. Take some object, say a small cross, and hold it 

 upright before the hole in S". It will cast a shadow in the 

 circular lighted area on screen 8\ and this shadow will be 

 upright. There is no reason why it should be otherwise. If, 

 now, a double convex lens is held behind the hole in screen 8'\ 

 the size of the lighted area and the cruciform shadow on 8' 

 will be altered, but the shadow will still be upright. 



NoAv for our experiment. Construct of thin pieces of wood 

 a frame like that shown in the drawing. The distance AB 

 is about 7 inches; the hole C is about one-tenth of an inch in 

 diameter, and DE is piece of white letter paper about 

 4 inches square, pasted over the wooden upright at the 

 left. At i^ a tiny hole is pierced through the paper with an 



♦■Reprinted from the Scientific American, June 3, 1911. 



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