CHAMBERS'S INFORMATION FOR THE PEOPLE. 





from the entrance of the optic nerve nearly as 

 far as the lens. On examining the retina at the 

 back of the eye by an instrument called an 

 ophthalmoscope, we observe, directly in a line 

 with the axis of the globe, a circular yellow spot 

 called, after its discoverer, the yellow spot of 

 Sommering. The only mammals in which it 

 exists are man and the monkey. It is the point 

 of distinct vision. When we read a book, we 

 run the eye along the lines so as to bring portions 

 of the line successively on the yellow spot. If, on 

 the other hand, we carefully fix our attention on a 

 word in the middle of the line, we see the word 

 distinctly, because it is on the yellow spot, while 

 the words towards each end of the line are less 

 distinct, being on other portions of the retina. 



The structure of the retina, as revealed by the 

 microscope, is seen in fig. .9. 



The transparent media through which rays of 

 light must pass before they form on the retina the 

 images of external objects are : 



Immediately behind the transparent cornea is the 

 aqueous humour, which fills up the chamber be- 

 tween the cornea and the lens. It is nearly pure 

 water, with a trace of chloride of sodium. 



The crystalline lens lies opposite to and behind 

 the pupil, close to the iris, and its posterior surface 

 is received into a depression on the forepart of 

 the vitreous humour (see fig. 8). In form, it is a 

 double-convex lens, with surfaces of unequal cur- 

 vature, the posterior being the most convex, and 

 the curvature is also less at the centre than 

 towards the margin. 



The vitreous humour lies in the concavity of the 

 retina, and occupies about four-fifths of the eye 

 posteriorly. 



The appendages of the eye are : 



1. The muscles by which the eye is moved are four 

 straight (or rectt) muscles, and two oblique (the 

 superior and inferior). By the duly associated 

 action of these muscles, the eye is enabled to move 

 (within definite limits) in every direction. 



2. The eyelids are two thin movable folds placed 

 in front of the eye, to shield it from too strong 

 light, and to protect its anterior surface. The 

 eyelashes intercept the entrance of foreign par- 

 ticles directed against the eye, and assist in 

 shading that organ from an excess of light. 



3. The lachrymal apparatus consists of the lach- 

 rymal gland, by which the tears are secreted ; 

 two canals, into which the tears are received near 

 the inner angle of the eye ; the sac, into which 

 these canals open ; and the duct, through which 

 the tears pass from the sac into the nose. The 

 constant motion of the upper eyelid induces a 

 continuous gentle current of tears over the surface, 

 which carry away any foreign particle that may 

 have been deposited on it. 



The various uses of the different structures of 

 the eye are readily understood. Assuming a 

 general knowledge of the ordinary laws of geo- 

 metrical optics, we will trace the course of the 

 rays of light proceeding from any luminous body 

 through the different media on which they impinge. 

 If a luminous object, as, for example, a lighted 

 candle, be placed at about the ordinary distance 

 of distinct vision (about ten inches) from the front 

 of the eye, some rays fall on the sclerotic, and 

 being reflected, take no part in vision ; the more 

 central ones' fall upon the cornea, and of these 

 some also are reflected, giving to the surface of 



126 



the eye its beautiful glistening appearance ; while 

 others pass through it, are converged by it, and 

 enter the aqueous humour, which probably, also, 

 slightly converges them. Those which fall on 

 and pass through the outer or circumferential part 

 of the cornea are stopped by the iris, and are 

 either reflected or absorbed by it ; while those 

 which fall upon its more central part pass through 

 the pupil. The rays now impinge upon the lens, 

 which, by the convexity of its surface, and by its 

 greater density towards the centre, very much 

 increases the convergence of the rays passing 

 through it. They then traverse the vitreous 

 humour, whose principal use appears to be to 

 afford support to the expanded retina, and are 

 brought to a focus upon that tunic, forming there 

 an exact, but inverted image of the object. 



Accommodation of the Eye to Distance. -It will 

 be found on experiment that we cannot see a dis- 

 tant and a near object at the same moment. For 

 example, if we look through a railing at a distant 

 church spire, and fix our attention on the spire, 

 we do not distinctly see the railing ; and vice versa. 

 This was early observed ; but, until recently, the 

 mechanism by which the eye accommodates or 

 focuses itself for different distances was unknown. 

 Cramer was the first to point out that if we bring 

 a candle-flame near the eye in a dark room, we 

 may see three images ist, an erect image reflected 

 on the cornea ; ad, an erect image on the anterior 

 surface of the lens ; and 3d, an inverted and very 

 faint image on the posterior surface of the lens. 

 He also shewed that when the eye looks quickly 

 at a near object, after having been for some time 

 directed to a distant one, the middle image moves 

 forward nearer to the first, and also becomes 

 smaller, shewing that for near vision the anterior 

 surface of the lens becomes more convex. Helm- 

 holtz afterwards, by means of an instrument called 

 the ophthalmometer, measured the sizes of those 

 reflections, and, from certain data, calculated by 

 mathematical formulae the radii of curvature of 

 the reflecting surfaces ; and he shewed conclusively 

 that the accommodation of the eye for different 

 distances is effected by changes in the curvature 

 of the anterior surface of the lens. The physio- 

 logical explanation is as follows : 



The lens, which is elastic, is kept habitually in 

 a state of tension by the pressure of the suspensory 

 ligament, and consequently has a flatter form than 

 it would take if left to itself. When the ciliary 

 muscle contracts, it relaxes the ligament, and 

 thereby diminishes its elastic tension upon the lens. 

 The lens, consequently, becomes more convex, 

 returning to its former shape when the ciliary 

 muscle ceases to contract. 



There are two common forms of defective vision 

 which require notice namely, short-sightedness 

 or myopia, and long-sightedness or presbyopia. 

 They are due to an abnormality either in the 

 curves or in the density of the refracting media. 

 In short-sightedness from too great a refractive 

 power from either cause, the rays from objects at 

 the ordinary range of distinct vision are brought 

 too soon to a focus, so as to cross one another, 

 and to diverge before they fall on the retina ; the 

 eye in this case being able to bring to the proper 

 focus on the retina only those rays which were 

 previously diverging at a large angle from a very 

 near object. The correction for this deficiency 

 is accomplished by interposing between the eye 



