116 ELEMENTARY PHYSIOLOGY. XXIII. 



5. Accommodation and inversion of retinal 

 image. (Schemer's experiment.) 



Sit facing a window. Fix a needle vertically about 

 12 inches in front of you. Look through the holes as 



in 4. 







a. Look at some distant object, two blurred images 

 of the needle will be seen; slide the nail of the middle 

 finger along the card, and block out the right-hand hole, 

 the blurred image of the left-hand side will disappear. 



The rays passing through the two holes have not 

 united when they fall on the retina. Thus the image on 

 the right side of the retina is cut out, and this is referred 

 to the left side. 



b. Hold the point of a second needle a little in front 

 of the card, and look at this; when it is distinct, the 

 other needle will become blurred and appear as two. 

 Block out the right-hand hole, the right-hand image 

 will disappear. Here the rays passing through the two 

 holes have united and crossed before they fall on the 

 retina, so that blocking the right-hand hole cuts out the 

 image on the left side of the retina, and this is referred 

 to the opposite, i.e. to the right side. 



6. Inversion of retinal image. Hold a card with a 

 pin hole a short distance from the eye. Move the head 

 of a pin from below upwards, between the eye and the 

 pin hole ; the head of the pin will be seen inverted. The 

 pin casts a shadow on the retina; the shadow on the 

 lower part of the retina is referred to the upper part of 

 the field of vision. 



7. Helmholtz's Phakoscope. This should be used 

 in a dark room. A looks with one eye through the hole 



