THE SKIN. 173 



lens is like the glass lens in the end of the tube. The 

 lining of the cavity of the eye-ball is colored dark, like 

 the inside of the box of the camera. The retina is like 

 the sensitive plate which the photographer puts in just 

 before he takes the picture. The brain behind the eye 

 is represented by the photographer himself looking 

 through from behind his instrument. 



10, When we look at an object, a picture of it is made 

 on the retina. This picture can be seen by a skillful ob- 

 server looking into the eye with an instrument called an 

 ophthalmoscope. It may be seen by looking from behind 

 through an eye taken from an animal just killed, as the 

 photographer looks through his camera. 



The retina, which is the ending of the optic nerve, 

 carries in to the brain the impression which makes the 

 picture. It is not the eye that sees. If the optic nerve is 

 cut off, we do not see; although a picture will still be 

 formed on the retina as before. It is the brain that sees. 

 The eye is its instrument. 



CAUSES OP TROUBLE IN THE EYE. 



11, The eye is sometimes weak, and gets very tired 

 and sore by use, simply because the whole body is weak. 

 But generally when there is redness, swelling, or pain in 

 the eye, there is something wrong in itself. It may be 

 that the delicate lining of the lids, the conjunctiva, is 

 inflamed, just as the lining of the throat is inflamed at 

 times. The lids then feel rough, as if there were sand 

 under them. 



TOO LONG SIGHT. 



12, One of the commonest causes of aching eyes and 

 head, after reading, is too long sight. When we look at a 



