220 



EXPERIMENTAL PIIYSinUx; V 



for the observer's eye, and at the third another aperture through which the ob- 

 served eye looks straight forward through an aperture above c. The observed 

 first looks at a far object and the images are then noted, especially with re- 

 ^ = ^____ gard to their size and position. The images 



^a^l obtained are given in fig. 172 a. The two 



bright ones to the left are from the ante- 

 rior surface of the cornea, the lower one 

 being formed from b, the upper from V. 

 The middle pair are much larger than the 

 first, and the third pair are smaller than 

 any, and inverted. Prove this by blocking 

 b' with a card. The lower image of the 

 right-hand pair and the upper images of 

 the other two pairs disappear 



The observed eye is now accommo- 

 dated for a near object by looking at a 

 pin in the shutter c, when the images 



el I m 



Fig. 171.— The Phakoscope. 

 (McKendrick.) 



Fig. 172. — The Reflected Images as seen in 

 the Phakoscope : a, while the Eye is at Rest ; 

 b, during Accommodation. (McKendrick.) 



change to those seen in b, fig. 1 72. The middle pair become smaller, lie 

 closer to one another, and approach the first pair. The third pair separate a 

 little from one another and become a little larger. 



The changes in the media of the eye, brought about during ac- 

 commodation, are further illustrated by the following experiment, 

 known as 



Scheiner's Experiment. — Take a long strip of wood, and to one end 

 fix a card vertically, and pierce this with two fine pin-holes lying close 

 together and on a horizontal line. They must be so near each other that 

 they both lie within the diameter of the pupil. Fix two needles vertically 

 on the wood, one about eight inches in front of the card, the other about 

 twenty-four inches away. Close one eye and look through the pin-holes in 

 the card at the two needles. 



1. Fix the eye upon the distant needle. A clear single image is obtained 

 of this, but two blurred images of the nearer needle are at the same time ob- 

 served. Now close the right-hand pin-hole, when the left-hand image of the 

 near needle disappears. On closing the left-hand pin-hole the right-hand 

 image disappears. 



2. In the second place look at the nearer needle, when a double image of 

 the far needle will be observed. Now close the right-hand pin-hole, when 

 the right-hand image disappears. On closing the left-hand pin-hole, the 

 left-hand image disappears. 



The meaning of this experiment will become clear from a study of 



