SCHEINER'S EXPERIMENT 



221 



fig. 173. i illustrates the condition of things in the first part of the 

 experiment, c c 1 is the card, a and b the two pin-holes ; P is the far 

 needle in transverse section, and r the near needle. The eye is 

 accommodated for the far needle r, and the rays of light from it (e.g. 

 the continuous lines of the figure) which pass through A and B meet 

 on the retina at y. Hence there is a clear single image of P. The 

 light from r (i.e. the interrupted lines of the figure) are not, with the 

 position of the refracting surfaces, sufficiently refracted to meet on the 

 retina, and consequently fall upon it in two patches at r 1 and r-. 

 These are able, however, to give a moderately good image, because 

 they are only formed from the rays passing through two minute 



Fig. 173. — To Illustrate Schkinek's Experiment. 



orifices, a andB. On now closing the left-hand pin-hole, a, the image 

 at r 1 disappears. This lies on the retina to the left of the second 

 image at r 2 . But the left half of the retina is normally concerned 

 with objects lying to the right, and vice versa. Consequently the 

 mind projects the image at r\ as if it were coming from an object to 

 the right, and the image at r 2 as from an object to the left. Blocking 

 a therefore causes the right-hand image to disappear, and blocking B 

 the left-hand image. 



ii gives the condition of things for the second half of the experi- 

 ment. The eye is accommodated for B, and the rays are more 



