CHAMBERS'S INFORMATION FOR THE PEOPLE. 



the lens from A are all very nearly brought to 

 a focus at this point. In this way each point 

 of the object has its image thus formed, and 



Fig. 1 6. 



consequently an image of the object is pro- 

 duced, but inverted. The image is also slightly 

 curved, as the points a, b, c, are not necessarily 

 in a straight line. The relative size of the object 

 and image will depend on the distance of the 

 former from the lens. If this distance is twice the 

 focal length, the size of the image is the same as 

 that of the object ; and if the distance be greater 

 than this, the image is less than the object in the 

 proportion of their distances from the lens. If the 

 object ABC be the sun, the rays from it falling 

 on the lens may be considered parallel, and there- 

 fore its image is formed in the principal focus. 

 This gives us a method of practically finding the 

 focal length of a lens. Receive the sun's image on 

 a screen of ground glass, or tissue-paper, and by 

 moving the lens or the screen backwards and for- 

 wards, with their plane always at right angles to the 

 sun's rays, form as distinct and small a circle of 

 light as possible : measure the distance of the lens 

 from the screen, and this is the focal length of the 

 lens. 



THE EYE VISION. 



We can now understand the action of a lens 

 in assisting imperfect vision. As mentioned in 

 the article HUMAN PHYSIOLOGY, the eye, in 

 front, consists of the iris, or variously coloured 

 ring, which has the property of contracting or 

 expanding to regulate the admission of light 

 through the little dark spot in the centre called 

 the pupil. Immediately behind the iris and pupil, 

 there is a transparent substance resembling in 

 shape a double convex glass, which is thence 

 called the crystalline lens. The use of this lens is 

 to collect and refract the rays of light, so that they 

 may converge to a point beyond ; in other words, 

 cause them to fall on the back part of the eye, 

 called the retina. Such are the main instruments 

 of vision ; and the sense of seeing is produced by 

 certain nerves which convey intelligence of the 

 image on the retina to the brain. If these nerves 

 be injured, the image will still be pictured on the 

 retina, but the mind will possess no power of 

 recognising their presence. 



It will be understood from these explanations 

 that the main instrument of vision is the crystal- 

 line lens, which collects the rays, and brings them 

 to a focus on the retina. If the lens be perfectly 

 transparent, and of the proper convexity, the light 

 is enabled to act with due effect on the retina, and 

 the representation of the object looked at will be 

 correctly pictured to the mind. But if the trans- 



246 



parent coating of the eye be dull, or the lens be 

 either too flat or too convex, every object will 

 appear dim. 



Two kinds of defective vision are more common 

 than any other, and they are known by the name 

 of long-sightedness and short-sightedness. Long- 

 sightedness, or the power of seeing objects best at 

 a considerable distance, is caused by too great a 

 flatness in the crystalline lens and outer coating of 

 the eye ; and the deficiency of vision in old persons 

 is usually from a similar cause. To remedy this 

 defect as far as possible, artificial lenses of glass 

 are employed. These lenses are called spectacles, 

 and act in the manner we are now to describe. 

 The following figure represents an eye in which the 

 crystalline lens is too flat. CA is the cornea or 

 outer covering, b is the crystalline lens, and d is 

 the retina behind : B is the object looked at. We 



Fig. 17. 



may observe, that in consequence of the flatness of 

 the lens b, the rays proceeding from the object 

 are not sufficiently refracted, but proceed to a focus 

 as far back as R ; in other words, the focus would 

 be at R, if the retina would permit ; but as the 

 retina is in the way, the rays, from not being focal- 

 ised upon it, cause imperfection in the vision. To 

 remedy this, we interpose an artificial convex lens, 

 or glass of a pair of spectacles, mn, and by its aid 

 the rays represented by dotted lines in the figure 

 are brought to a focus on the retina at d. Thus, 

 by selecting spectacles of a proper focalising power 

 in relation to the eyes, one kind of imperfect vision 

 is very happily remedied. 



Short-sightedness arises from a cause the reverse 

 of that just alluded to, being produced by too great 

 a degree of convexity in the crystalline lens and 

 cornea. In this case the rays come to a focus too 

 soon within the eye, and do not reach the retina, 

 unless the object is brought quite close to the 



Fig. 18. 



organs of vision. We here offer a representation 

 of this condition. In consequence of the project- 

 ing globularity of the cornea, CA, and the too great 

 refracting power of the crystalline lens, the rays 

 from the object B fall short of the retina at R. 

 To remedy this, we interpose a double concave 

 lens, LL, by which the rays are rendered more 



