“The retina, my dear, from a Latin word that means anything by which another thing is held or retained, as this part of the eye holds or retains the picture of things. You ask me what is between the pupil and the retina. There are in the eye three different substances, called humours, all transparent. A transparent substance means anything that can be seen through. The first one, directly back of the pupil, is called the acqueous, from a Latin word, that means watery: it is a thin liquid, like water. The second, behind that, is called the crystalline humour, from its clearness and brightness. It is formed like the glasses they use in telescopes, and is fastened at the edge by the delicate transparent substance that covers it, called a membrane. The one beyond this, and next the retina, is called the vitreous humour, from its resemblance to glass. All these substances assist in forming the images of objects on the back of the eye; but you are not old enough to understand how, at present, my child.”
“Then, mother,” said Catherine, “our eyes are as curious as grandfather’s telescope, or as the camera obscura, that he gave us to see pictures with.”
“They are far more curious my dear; and it is by imitating the eye that they can make them so well. I remember, Catherine, when your grandfather sent that camera obscura to you the other day, and your father showed you the pictures in it, that you and Lucy and James capered about the room with joy, saying, ‘Oh, how good grandfather is to give us such a beautiful thing!’—and now, my dear, when you go into a garden and dance with joy at the sight of the flowers; when you look up with so much wonder and delight at the beautiful moon sailing through the clouds, and at the bright twinkling stars; when, after having been even one day away from your father and mother, you feel so happy at looking in our faces, and reading in them our love for you,—of whose goodness ought you to think? Who has given you eyes to see all these delightful things? Whom should you then love? Of whom then should you speak, and say, ‘Oh, how good He is?’”
Catherine felt and understood what her mother said, and answered her, that it was God.
“I have yet much more, my dear,” said her mother, “to tell you about the eyes, that is very wonderful. This beautiful little round window grows larger and smaller as you want more or less light. When there is a great light, it contracts so as to take in but little; and when the light is faint, it becomes nearly twice as large, so as to take in more.”
“Why, mother,” said Catherine, “how can that be?”
“Shut the shutter,” said Mrs. Nelson, “and then look in my eye.” She did so; and she saw the pupil of her mother’s eye grow larger and larger. “Now open it,” said her mother. She did so, and it gradually became smaller. “Oh, it is very curious,” said Catherine. “But, mother, is not that pretty rim round the pupil of any use?”
“That is what it called the iris,” answered her mother, “which is the latin name for rainbow, I suppose from some fancied resemblance to it. It is thought that by means of it the pupil of the eye is enlarged or contracted. If you remember, my child, the pain you feel in your eyes when you come from the dark suddenly into the light, you will understand the use of this, and see what a beautiful contrivance it is. In the dark your pupils become very large, so as to catch all the light they can. When the light comes before they have time to grow smaller, they take in more light than they can bear without pain.