Feb. 23, 1883.] 



♦ KNOWLEDGE 



111 



HOW TO USE OUR EYES. 



By Joun Buowxisii, F.R.A.S. 



IT might at first thought appear that it cannot be 

 necessary for any pi.Tson to learn to see. A moment's 

 consideration will show that this is a mistake. Before an 

 artist can draw any object well he must be able to see 

 the most delicate light.s and shades upon its surface. I 

 have often been surprised at the remark that " a photo- 

 graph is much more beautiful than the landscape it was 

 taken from." Now I know the speaker coiihl not see the 

 landscape. 



Tt is stated that a lady observed to the great artist, 

 Turner, pointing to one of his pictures, " Dear me ! Mr. 

 Turner, I never saw anything like that ! " " No, madam," 

 answered Turner ; " don't you wish you could ! " 



Those who are in the habit of using microscopes or 

 telescopes can see minute details or an object which would 

 not be visible to a person who looked through one of those 

 instruments for the first time. The remark is frequently 

 heard that a particular person is an excellent judge of 

 some article. It will generally be found that the person 

 can see diflerences in various samples of the article which 

 are not visible to most people. 



A good photograph of a statue appears almost solid. 

 This is due entirely to the delicate shades being faithfully 

 represented in the photograph. There are but few persons 

 whose sight would be keen enough to enable them to detect 

 the whole of these shades on the original statue. This is 

 the principal reason that a drawing, however beautiful or 

 truthful, never looks as solid as a photograph. 



SCLEROTIC 

 CHOROID 



fteTiN&' 



V TENDON OF RECTUS 



CILIARV WUSCLS 

 &LICAMEr(T 

 ANTERIOR 

 CHAMBER 



HYALOID- 



MEMBRANl 



Fig. 1. 



Fig. 1 is a diagram of the eye, wliich shows the Cornea, 

 the Iris, the Ciliary Muscle, the Crystalline Lens, the 

 Aqueous Humour, the Retina, and the Choroid Coat. 



The outer wliite coating of the eye is called the sclerotic. 

 The central portion of the eye is known as the cornea ; 

 behind this is the iris, the coloured portion of the eye. 

 There is an opening in the centre of the iris known as the 

 pupil ; this is simply an aperture through which light can 

 pass. Thi.s aperture opens when the eye is in a faint light, 

 and closes when it is exposed to a strong light. The space 

 between the cornea and the iris is tilled with a watery 

 fluid. Just behind the iris is the crystalline lens. This 

 is popularly supposed to be the pupil of the eye, and you 

 have doubtless heard the expression, having the pupil taken 

 out of the eye. Kow, you could no more take the pupil 

 out of an eye than you could take a keyhole out of a door, 

 as it is simply an opening which admits light into the 

 crystalline lens. 



Outside the iris is the ciliary muscle, a ring of muscular 

 fibre. This muscle makes the crystalline lens more convex 

 whenever we look at an object witliin a few feet of us. 



The whole space of the eyeball is filled with the vitreous 

 humour, a jelly-like substance. 



The back part of the inside of the eyeball is covered 

 with a most exquisitely sensitive nerve tissue, known as 

 the retina. On this retina a picture of any object in front 

 of the eye is formed. The optic nerve extends from the 

 brain to the back of the eye, and small branches of it 

 extend all over the retina. These convey to the brain the 

 information as to the kind of picture which is formed on 

 the retina. Outside the sensitive surface or retina there 

 is a coating of black pigment, known as the choroid 

 coat, which serves as a background to stop rays of light 

 which might otherwise pass through the retina. 



There are several large muscles round the outside of the 

 eyeball. By means of these the eye may be moved to 

 either side or upwards or downwards, so as to get a clear 

 view of any object. 



Fig. 2. 



Fig. 2 is not an anatomical, but a rough mechanical 

 and optical model of the eye. It is made of an opal lamp 

 globe, with large openings at the top and bottom. On one 

 side is a common bidl's-eye ; this represents the crystalline 

 lens. The other opening is covered with a piece of partially 

 transparent tracing-paper. This receives the image formed 

 by the buU's-eye or crystalline lens. It will be seen that 

 the letter A appears reversed ; in simple language, it is 

 upside down. 8o are the images of the objects we see on 

 our retina ; they are all upside down. 



One of my scientific friends, the brother of one of our 

 most distinguished musical composers, has taught himself 

 to read print when held upside down. Occasionally he 

 will read a book in this maimer when he is travelling in an 

 omnibus or a railway-carriage. It is not long bcfor(! some 

 passenger draws his attention to the fact that he is holding 

 the book the wrong way. Their astonishment is great 

 when he quietly informs them that he prefers reading 

 with the book held in that direction. I am afraid tliat 

 occasionally they have^ doubts of my friend's sanity. 

 If I have explained the action of the eye with sufficient 

 clearness, you will at once understand that the letters of 

 the book held upside down really appear upon my friend's 

 retina in an erect position. 



Sometimes the question is put to me, " Will you believe 

 your own eyes 1 " To this I reply, " As an optician, 

 certainly not." 



Numerous ways in which the eyes can be deceived are 

 no doubt familar to you. The best-known of all is, 

 perhaps, that known as Pepper's Ghost. In this optical 

 illusion a number of persons appear to be upon a stage or 

 platform in front of the audience ; in reality they are 

 before the stage, but out of view of the audience. 



How easily our eyes may be deceived may be proved 

 by a very simple experiment which you may make. Take 

 a large card — the size is of no consequence. Make a large 

 black circular spot on the card, on the right-hand side, one 

 inch in diameter, then at a distance of three inches from it, 

 on the left-hand side, make a black dot the size of a pin's 

 head. If you hold this card at exactly one foot from your 

 right eye and look intently at the small dot, the large black 



