1.The best way to study the more important parts of this very beautiful and wonderful organ，is to get a bullock‘s eye from the butcher and examine it well.In front you will see a transparent part，like the glass of a watch.This is called the cornea ；it is thewindow of the eye.Through it you can see the pupil ，the dark portion in the centre，and the brown iris which surrounds it.The cornea extends to the sclerotic ，or white of the eye；and you will find that the white and the cornea together enclose the eye in a protecting coat，through which the nerve of the eye，called the optic nerve ，enters from behind.
2.If you get a sharp pair of scissors and cut through the white，just behind the cornea，so that the front part of the eye is separated from the back portion，you will be able to study the inside.Looking first at the frontpart，you will see the lensof the eye just behind theiris.It is about half an inch across，or perhaps a littlemore，and it appears to be made of the purest and most brilliant crystal.If you remove the lens，you will see the iris，which now looks like a circular curtain，while the pupil is nothing but a hole in its centre.In the eye of the ox and of the cat the pupil is a slit，but in the human eye it is a round hole.
3.If you watch the eye of a friend，you will see that the pupil changes its size：when there is much light，the pupil is small ；but when there is little light，or immediately after the eyes have been closed，the pupil is large.The iris has，therefore，the power of changing its shape，so that the hole in its centre may become small when there is much light，and large when there is little.
4.If you now look at the remaining half of the bullock’s eye，you will observe two thin coats insidethe white.The first is a black layer which at some parts has a glistening green colour；this is called the choroid coat.The second is very thin，yellow in colour，and so delicate that you will probably have torn it with your scissors；this is called the retina .It is full of nerves，which all run to the back of the eye，unite to form the optic nerve，and then pass to the brain.
5.When we look at an object，say a piece of white paper，the rays of light from the paper pass through the transparent cornea and the pupil，and are brought to a focus by the lens，so as to form an image upon the retina.This image affects the nerves of the retina，and these affect the brain，and we are conscious of the white paper.It is quite easy to see the image that the lens is capable of making.You may set about it in the following way：-Get a pill-box，remove the top，cut out a hole inthe bottom into which you can fit the lens，and fasten itthere.Take a piece of thin white paper，rubbed with oil or grease to make it transparent，and hold it over the top of the pill-box.Then turn the box with the lens towards a lighted candle or a window，and look from behind at the greased paper.You will see upon it a tiny image of the candle or of the window turned upside down.
6.As we have seen，the retina receives the images which are cast upon it；but all parts of it are not equally sensitive.When we look at the middle of a page ofprint the letters there are seen quite clearly，but at theside they are blurredand indistinct.When we wish tosee anything，we turn the eye so that its image may fall on the centre of the retina.But very near the middleof the retina there is a small spot which is absolutelyblind；this is called the blind spot .Make a dot on a piece of white paper，and about three inches to the right of this draw a cross.Now close the left eye，lookat the dot with the right eye，and holding the paper some ten inches in front of the face，move it about a little，always keeping the eye fixed on the dot.In most positions both the dot and the cross are seen；but in one position the image of the cross is cast upon the blind spot，and the cross suddenly disappears.
7.The lens-so transparent that it is generally called the “crystalline”lens-frequently becomes clouded in old age.The eye then becomes useless，for the light cannot pass through it，and we say that a cataract has formed.When this cataract is to be removed by an operation，the eye-surgeon cuts into the cornea and removes the lens；and although the patient‘s sight is never again perfect，he can see fairly well with the help of spectacles.
8.This most delicate organ is often much abused，and in consequence many defects of sight arise.There is nothing so bad for the eyes as writing，reading，sewing，or doing any fine work，in a dim light.School-rooms should have large and high windows，and the scholars should be so placed that the shadows of their bodies donot fall upon the paper with which they are working.Both in reading and in writing the head should be held well up，and should never be bent over the work，for then the eyes become too full of blood，and fatigue rapidly comes on.One cannot be too careful of the eyes in youth，for no loss is greater than that of sight.