PHYSIOLOGY FOR BEGINNERS 



of the retina itself, and it is all the same to us whether all the 

 images taken together on the retina be inverted or no. 



Accommodation. Put two needles upright into a board, 

 one about 10 inches from the near end, and the other about 18 

 inches farther away. Close one eye, and with the other eye 

 close to the end of the board, and almost in a line with the 

 needles, look fixedly at the distant needle ; it is seen clearly 

 because when you look at it a clear image of it is formed on 

 the retina ; at the same time the near needle will be seen, 

 but only indistinctly ; it will appear not clear but blurred. 

 Similarly, if the near needle is looked at fixedly, a clear 

 image of it is formed on the retina, and at the same 

 time a blurred and indistinct image of the distant needle. 

 Certain changes can take place in the eye, so that it can be 

 accommodated for the near needle or for the distant needle, 

 but not for both at the same time. The objects we see may 

 be at any distance from the eye, between a quite far distance, 

 such as the horizon, and a short distance of about 10 inches, 

 which is usually as near as things are held by most people. 

 Throughout this wide range, and even up to 5 or 6 inches 

 from the eye, we see things distinctly when we look at them, 

 that is to say, clear images of objects are formed on the retina. 

 Since no change in the distance of the screen, the retina, from 

 the crystalline lens, can take place, this accommodation of the 

 eye for objects at a varying distance can only be effected by 

 changing the strength of the crystalline lens, that is, by 

 changing its curvature, and it can be shown that this actually 

 does take place. We might, it is true, gain the same result 

 by changing the curvature of the cornea, but this does not 

 take place. When the distant needle is looked at, in the 

 experiment just mentioned, only a blurred image of the near 

 needle is obtained, because the rays from the latter reach the 

 retina before they are brought to a focus. This may be shown 

 in the following way. With a needle prick two holes in a card 

 about one-sixteenth of an inch apart, that is, closer together 

 than the diameter of the pupil, so that you can look through 

 both holes at once. Fix the card, with the holes horizontal, 

 upright to the end of the board, 8 to 10 inches from the 

 nearer needle. Hold the board to the window or facing a 

 good light, close one eye, and with the other look through the 



