604: HANDBOOK OF PHYSIOLOGY. 



As an optical instrument., the eye is superior to the camera in the fol- 

 lowing, among many other particulars, which may be enumerated in de- 

 tail. 1. The correctness of images even in a large field of view. 2. 

 The simplicity and efficiency of the means by which chromatic aberra- 

 tion is avoided. 3. The perfect efficiency of its adaptation to different 

 distances. In the photographic camera, it is well known that only a 

 comparatively small object can be accurately focussed. In the photo- 

 graph of a large object near at hand, the upper and lower limits are 

 always more or less hazy, and vertical lines appear curved. This is due 

 to the fact that the image produced by a convex lens is really slightly 

 curved and can only be received without distortion on a slightly curved 

 concave screen, hence the distortion on a flat surface of ground glass. 

 It is different with the eye, since it possesses a concave background, 

 upon which the field of vision is depicted, and with which the curved 

 form of the image coincides exactly. Thus, the defect of the camera 

 obscura is entirely avoided; for the eye is able 1o embrace a large field of 

 vision, the margins of which are depicted distinctly and without distor- 

 tion. If the retina had a plane surface like the ground glass plate in a 

 camera, it must necessarily be much larger than is really the case if we 

 were to see as much; moreover, the central portion of the field of vision 

 alone would give a good clear picture. (Bernstein.) 



B. Defective Accommodation Presbyopia. This condition is 

 due to the gradual loss of the power of accommodation which is part 

 of the general decay of old age. In consequence the patient would be 

 obliged in reading to hold his book further and further away in order to 

 focus the letters, till at last the letters are held too far for distinct vision. 

 The defect is remedied by weak convex glasses, which are very commonly 

 worn by old people. It is due chiefly to the gradual increase in density 

 of the lens, which is unable to swell out and become convex when near 

 objects are looked at, and also to a weakening of the ciliary muscle, and 

 a general loss of elasticity in the parts concerned in the mechanism. 



VISUAL SENSATION'S. 



Excitation of the Retina. Light is the normal agent in the ex- 

 citation of the retina. The only layer of the retina capable of reacting 

 to the stimulus is the rods and cones. The proofs of this statement may 

 be summed up thus : 



(1.) The point of entrance of the optic nerve into the retina, where 

 the rods and cones are absent, is insensitive to light and is called the 

 blind spot. The phenomenon itself is very readily demonstrated. If 

 we direct one eye, the other being closed, upon a point at such a distance 

 to the side of any object, that the image of the latter must fall upon the 

 retina at the point of entrance of the optic nerve, this image is lost either 

 instantaneously, or very soon. If, for example, we close the left eye, 

 and direct the axis of the right eye steadily towards the circular spot 

 here represented, while the page is held at a distance of about six inches 



