22 ORGANS OF THE SENSES THE EYE DIAGRAM 2. 



persons. These tints are also sometimes slightly green, or 

 yellowish. If we look at the eyes of the same person in the 

 sun and in a dark place, we see that the pupil is not always of 

 the same size, it enlarges in the shade, and contracts in full 

 daylight. This is particularly noticeable in cats. We have 

 only to look at their eye in the sunlight to see the pupil reduced 

 to a narrow vertical line, not the tenth part of an inch in 

 breadth, which no longer occupies the whole height of the eye. 

 In the evening, and especially on a dark night, the pupil en- 

 larges till the iris can be no longer distinguished, or is only 

 visible as a narrow border all round the eye. 



In front of the eye is a convex transparent part extended 

 before the iris ; this is the cornea,. Behind the pupil, and con- 

 sequently behind the iris, is an organ like a magnifying glass, 

 and as transparent as crystal, which is called the crystalline lens. 

 Behind this, the eye is filled with a kind of transparent jelly, 

 the aqueous humour. Lastly, the back of the eye is curtained by 

 an extremely delicate nervous membrane, which is called the 

 retina. It is connected with a large nerve, which runs from the 

 back of the eye to the brain. External objects paint them- 

 selves on the retina through the pupil, and then we see them. 

 If the crystalline lens grows dim, sight is lost, which happens 

 in cataract. The eye can very well be compared to the apparatus 

 used by photographers, and called a camera obscura. In front 

 the object-glass represents the crystalline lens and the cornea. 

 At the back, external objects are painted on an unpolished glass, 

 which is entirely analogous to the retina. 



The eye, or as we say, the ball of the eye, moves in its orbit 

 to the right and left, and up and down, by means of muscles which 

 draw it in these four directions. When one of these muscles 

 is shorter than the three others, the eye is drawn to the side of 

 this muscle, and it is then said that the person squints. Every- 

 one does not see distinctly at the same distance. Some are 

 obliged to hold a book very close to their eyes to read, and 

 others are obliged to hold it at a distance. The first are said 



