1912'] 



Proof of Inveeted Images 



43 



ordinary pin. Now stick a pin upright in the block G and 

 adjust the position of the block so that the head of the pin is 

 exactly in line between the holes C and F, and three-fourths 

 of an inch from the hole C. Fix the block G in this position. 

 This completes our apparatus. Placing the eye close to the 

 hole C and looking through hole F at the sky, we see a lighted 



circular area with the shadow of the pinhead in its center, 

 but this shadow is inverted. We are ready to declare that 

 the pin is upside down, for it certainly looks so. When we 

 reflect a moment, however, we see that we have now exactly 

 the same arrangement as in the middle diagram. The hole F 

 represents the hole in the screen 8, the pinhead represents 

 the cross, the pupil of the eye represents the hole in screen 

 S'', and the retina of the eye takes the place of screen S' and 

 receives the upright shadow of the pinhead upon it. The 

 crystalline lens of the eye acts precisely like the lens in Fig. 

 2, altering the size of the retinal shadow, but not its upright 



