HO PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Fig. 78. 



form of the lens. For the complete proof of this we are 

 indebted to Helmholtz. The experiment is shown in 

 Fig. 77, in which A is the eye of the 

 patient observed, B the eye of the ob- 

 server, and C a candle. In looking into 

 the eye A with an ophthalmoscope three 

 images of the candle are distinctly seen 

 (Fig. 78). One of these — the first (a) — is 

 the reflection from the corneal surface. 

 The second (b), much fainter and smaller, 

 is from the anterior convex surface of the lens. Both of 

 these are upright. The third (<r), still fainter and smaller, 

 is inverted, because it is reflected from the posterior sur- 

 face of the crystalline, which is concave. All this is ob- 

 served while the patient gazes at a distant point (/, 

 Fig. 77). Now tell the patient to look at a very near 

 point («), perhaps six inches from the eye. Immediately 

 the image (6, Fig. 78) is seen to change. It becomes 

 smaller, and changes its place in such wise as to show 

 that the anterior surface of the lens has become more 

 convex, has bulged out, and even pushed the iris out a 



Fig. 79. — F, lens adjusted to distant objects ; jV, to near objects ; a, aqueous 

 humor ; rf, ciliary muscle ; e, ciliary process. 



little (see Fig. 79). Thus it seems certain that in accom- 

 modation of the eye to near vision the lens thickens and 

 becomes more refractive. But the question still remains, 

 How does it do this ? We are distinctly conscious of a 

 muscular strain. What muscle ? 



