THE EYE AND VISION 213 



and should wear convex glasses to assist the convergence of the 

 rays. 



It may now be asked, if the images of objects on the retina 

 are inverted, how is it that we do not see the objects themselves 

 inverted? In answer to this we may say that we do not see 

 with the retina, but with the brain. The rays of light make 

 impressions on the nerve-endings in the retina, but there is no 

 sense of vision until the disturbances in these nerve-terminations 

 are transmitted through the optic nerve and reach the brain, 

 which is the seat of all sensation. 



The retina is not equally sensitive in all parts. A certain 

 point almost exactly opposite the centre of the pupil is by far 

 the most sensitive to luminous impressions. It is marked by a 

 slight depression of surface, of a pale yellowish colour, and is 

 called the macula lutea, or yellow spot (fig. 190). When we 

 look at an object, that part of it is most distinctly defined in the 

 retinal image which is in a straight line with the yellow spot and 

 the centre of the pupil. When we wish, therefore, to see an 

 object as clearly as possible, it is necessary to turn the eyeballs so 

 that it shall be on this line the axis of vision. 



The optic nerve enters the back of the eyeball at a point about 

 ^th of an inch from the yellow spot on the nasal side. This 

 point is insensible to light, and is known as the blind spot. To 

 prove this, close the left eye, and look steadfastly on the left dot 

 below with the right, holding the page at a distance of about a 

 foot. 



Now move the book very slowly towards the eye, keeping it still 

 steadily fixed on the left dot. At a certain point (generally about 

 six inches from the eye) the right dot will suddenly disappear, 

 owing to its image falling on the blind spot ; but, on bringing the 

 book nearer, it will reappear as its image leaves this point. 



The retina has the power of retaining the impressions made 

 on it for a short time about th of a second. Consequently, the 

 falling raindrops often look like lines, the spokes of a rapidly re- 

 volving wheel do not hide objects behind them, and a lighted stick 

 moved very rapidly appears as a line of light. 



The sensibility of the retina is readily exhausted. Thus, 

 if we look at a very bright light for a time, that portion of the 

 retina on which its image falls soon becomes exhausted ; so that, 

 if we turn from the bright light and look on a sheet of white paper, 

 we see a dark spot. The rays of light from the apparently dark 

 spot fall on the part of the retina which has been exhausted. 



