VISION*. 241 



those which fall vertically near its centre; and therefore 

 the iris, by diminishing the pupil, prevents any such source 

 of error. 



Another difficulty in the manufacture of optical instru- 

 ments is to make them achromatic ; that is to say, to prevent 

 the different colours of which white light is made up from 

 being dispersed in passing through lenses, and so producing 

 rainbow colouring in the image. The mechanician meets 

 this difficulty by combining lenses of contrary form and 

 different material, in such a manner that the dispersion of a 

 converging lens is counteracted by the opposite and equal 

 dispersion of a diverging lens of less refracting power. The 

 same expedient is resorted to in the structure of the eye, the 

 light having to pass successively through the cornea, aqueous 

 humour, and the different strata of unequal density in the 

 crystalline lens. 



177. With reference to the correspondence of the position of 

 the retina with the focus of the lens, it will be noticed that 

 if that membrane be so placed that the images of distant 

 objects shall be cast on it, it will be too far forward to receive 

 the images of near objects; and, vice versd, if it be so situated 

 as to receive the images of near objects, it will be too far back 

 for those at a distance, unless some arrangement of accom- 

 modation be specially brought into play. In a photographer's 

 camera, the focus is arranged for different distances by moving 

 the lens forwards and backwards; in the eye, the same 

 object is attained by change in the form of the lens. 



It is easy to make certain that a change of some sort takes 

 place in the eye to accommodate it to different distances. 

 One has only to shut one eye, and hold up a finger a few 

 inches from the other, to perceive that, if the finger be 

 steadily looked at, the background, even at the other end of 

 the room, is quite indistinct; and, as soon as by an effort of 

 the will the sight is fixed on the background, the finger in 

 turn loses all distinctness of outline. But it is more difficult 

 to determine the nature of the change. It has been dis- 

 covered by careful observation of the reflections from the 

 surfaces of the lens (Helmholtz). "When a light is held in 

 front of an eye, three images are reflected; one is from the 

 surface of the cornea; another, of a dim description, is from 

 H Q 



