THE EYE. 39 



the opening called the pupil instead of being black 

 as midnight, has a chalky or white appearance. 



But the 'cataract' may not be complete, that is, 

 only part of the 'lens' may be white and opaque. 

 A * cataract' may be no larger than a pin's head, 

 and may be situated in any part of the ' lens.' 



30. — To test the lens we use our lighted candle 

 in our darkened stable. The test is called the 

 ' catoptric test,' and is very easily applied. You 

 take the candle and place it a little in front of the 

 eye, a few inches from it, when you see three images 

 of the candle-light; one upon the surface of the 

 * cornea,' one upon the front capsule of the ' lens,' 

 (our front watch-glass,) and the third still further 

 back, upon the hindmost capsule of the lens, (our 

 hindmost watch-glass). Now, after you have dis- 

 tinctly found these three images, notice that the 

 two front ones are upright like the candle-flame, but 

 the hindmost image is upside down. After quite 

 making out this fact, gently move your light from 

 side to side, and you will see that while the two 

 front upright images move in the same direction as 

 the candle, the hindmost one, which is turned up- 

 side down, moves in the contrar}^ way to the candle. 

 It is therefore evident that if the ' lens' is opaque 

 and milky you cannot see the hindmost or inverted 

 image, but you will only see the two foremost up- 

 right images. 



The cataract, as we have seen, may not involve 

 the whole lens, but may be just a little speck in 



