34 



THE POPULAE EDUCATOE. 



with a hole in the middle of it, which is opposite the middle of 

 the cornea, or window of the eye. 



From the same circle of attachment, but internal to the 

 curtain before-named, is suspended, or rather held, by a liga- 

 ment, a perfectly transparent body shaped like a lentil, that is, 

 with two convex but flattened surfaces. The quoit-like curtain 

 is called the iris, and the disc the crystalline lens. The lens is 

 slung at some little distance from the cornea, leaving a chamber, 

 which is filled with watery fluid, which bathes both sides of the 

 iris. Behind the lens, and occupying the larger part of the hollow 

 of the eye, is a denser liquid, contained in a thin, perfectly- 

 transparent membrane, which not only encircles it, but sends in 

 partitions from its outer wall to divide the liquid into compart- 

 ments, so that when the eye is cut into, the humour does not 

 run out, but seems to be of the consistence of clear jelly. Both 

 the liquid and capsule are so transparent that they are called 

 the hyaloid membrane and vitreous humour, or the glassy mem- 

 brane and humour. 



All the main parts of the eye have now been described except 

 the essential one for which all the others are made, namely, 

 the retina: that wonderful stratum of nervous matter which 

 receives and transmits to the brain all luminous impressions, 

 the glories of colour, the splendid imagery of the earth, and the 

 soft radiance of the sky. 



The retina lies between the choroid and vitreous humour. 

 It lines the choroid as closely as that membrane lines the 

 sclerotic, and so covers the whole back part of the eye. 



The retina (or sentient mirror), thin as it is, has been found 

 under the microscope to consist of many layers of diverse struc- 

 ture. Not to descend into great minuteness, it may be said to 

 consist of an outer layer of cylindrical bodies, called, from their 

 shape, rods and cones, which run perpendicularly to the surface 

 of junction between retina and choroid. These bodies are the 

 instruments by which the rays are noted. It would seem that 

 each rod or cone conveys but one impression, so that while the 

 image of an external object may be made very small on tho 

 retina, and yet distinctly seen, because of the minuteness of 

 these bodies, yet the image must cover a certain number of them 

 to be an image at all. In other words, if it only covered one, 

 the impression would be that of a single point of light. 



Next comes the granular layer, the office of which is no further 

 known than that similar structures are found wherever impres- 

 sions received by the senses are modified. The innermost layer 

 consists of nerve-fibres, which convey the impressions in some 

 such way as the telegraph wires convey their messages. These 

 all run to one point in the back part of the eyeball, a little on 

 the inner or nose side of the axis, and there pass through the 

 choroid and sclerotic, which are pierced by a great many holes, 

 and are united behind into the optic nerve, and this runs to the 

 brain, first, however, being joined by its fellow from the other 

 eye, and then separating from it again, having received some of 

 the strands of this nervous cord, and given up some of its own 

 in return. 



Let us now trace the course of a number of raya reflected from 

 a single point in an object, before they reach the retina (see Fig. 

 2). These rays as they come from a single point are, of course, 

 diverging. They strike, therefore, all over the surface of the 

 cornea, and as they pass through it are gathered somewhat 

 together. They then pass the aqueous humour with a slightly 

 altered course. The outer ones are cut off by the opaque iris, 

 but the central ones pass through the lens, which rapidly gathers 

 them together, and they are then transmitted through the 

 vitreous humour, all the time converging until they meet at a 

 point exactly in or on the retina. 



In saying that they meet exactly on the retina, it is meant 

 that they will do so if the adjustment is perfect. If it be im- 

 perfect, so that the rays unite in a point either before the retina, 

 or would unite behind it if they could traverse the choroid, the 

 image is blurred and indistinct. 



The problem of how to get a distinct image, of course, ia 

 more difficult, when the points from *"hieh the light proceeds 

 are numerous, as from any object of appreciable form. To ob- 

 tain this, the surface of the cornea, the hind and front face of 

 tho lens, and the face of the retina, must all be of definite and 

 regular curves, or the figure would be distorted. If the cornea 

 bulges too much, the object can only be seen at a short dis- 

 tance, and from this cause some persons have to lay their 

 'ueeks upon Jie page before they can read print. If it bulges 



too little, distinct images of near objects are impossible. It. 

 the crystalline lens is too dry, or too moist, it becomes clouded 

 with hard or soft cataract. If the pigment be not of sufficient 

 quantity in the choroid, vision is interfered with ; and from this 

 cause albinos, or persons whose hair and skin are deficient ia 

 colouring matter, are dazzled in ordinary daylight. 



Further, if the retina, or part of it fail, as it sometimes does, 

 from some cause too subtle to bo found out, the object is seen 

 only in part ; thus, some persons have this peculiar affection of 

 half the retina, so that when they look directly at an object, 

 they only see the half of it. 



The retina, perfect in all its other functions, may not dis- 

 criminate colour. The writer once played a game at croquet 

 with a gentleman, who disclosed his infirmity thus : Two balls 

 were lying together one red, and the other green. He asked 

 which was his, and being told the red one, asked which red 

 one ? On another occasion the writer was looking at a brightly- 

 coloured geological map. A stranger who looked with him soon 

 showed that he was quite unaware that it was other than the 

 ordinary ordnance map. 



These defects of vision call marked attention to the perfec- 

 tion of the instrument of vision, when perfect, as it is ia 

 most cases. 



It would be difficult to determine whether the eye were made 

 for light, or light for the eye ; but that the Creator of the one 

 was cognisant of all the wonderful qualities of the other, 

 admits of no doubt ; and this goes far to prove that the Creator 

 of the one must have been the designer of the other. 



LESSON'S IN ENGLISH. II. 



SIMPLE PROPOSITIONS. 



Alfred reads. 



THESE two words form what is called a proposition ; they form 

 a simple proposition. Proposition is a word of Latin origin, 

 signifying some^iing that is put before you. As being some- 

 thing that is put before you, it is a statement ; it is a statement 

 of a fact or a thought ; a statement of something in the mind, 

 or something 0ut of the mind. Here the statement is that 

 Alfred reads. Such a statement is also termed a sentence. 

 Sentence is also from the Latin, and signifies a ferm of words 

 comprising a thought or sentiment. These words, then namely, 

 sentence, proposition, and statement, have the same significa- 

 tion ; and they each denote an utterance, the utterance of a fact, 

 an idea, an emotion. Observe that both words are essential to 

 the proposition. Take away Alfred, you then have reads ; but 

 reads is no proposition, for nothing is stated. Take away reads, 

 you leave Alfred; but Alfred by itself says nothing, makes no 

 statement, and therefore forms no proposition or sentence. The 

 two words must concur to make a proposition. If so, less than 

 two words do not make a proposition ; and a proposition or 

 sentence may consist of not more than two words. 



In these simple statements you have in the germ the sub- 

 stance of the doctrine of sentences. If you understand what I 

 have now said, you have laid the foundation for a thorough 

 acquaintance with language in general, and with the English 

 language in particular ; for to a form of words similar in sim- 

 plicity to that which stands at the head of this lesson is all 

 speech reducible ; and that model presents the germ out of which 

 are evolved the long and involved sentences of our old English 

 divines, and the full and lofty eloquence of Milton's immortal 

 essay on behalf of the liberty of the press. 



The sentence as it stands is what is called an affirmative 

 proposition ; that is, it affirms or declares something it affirms 

 or declares that Alfred reads. The term affirmative is used in 

 opposition to the term negative. Negative propositions are 

 those in which something is denied. An affirmative may become 

 a negative proposition by the introduction of the adverb not ; 

 thus, Alfred reads not. In English it is more common to employ 

 also the emphatic does, as Alfred does not read. You thus see 

 that the words does (do, or dost, as may be required) and not 

 convert an affirmative into a negative proposition. Sentences 

 in which a question is asked we term interrogative ; as, does 

 Alfred read ? Here by the help of the emphatic form does, and 

 the inversion of the terms does and Alfred, we make an affirmative 

 into an interrogative sentence. If into this last sentence we 

 introduce the negative not, we have an interrogative negativa 



