124 CONSCIOUS NERVOUS OPERATIONS 



est and instruct his pupils, but confer lasting benefits upon some of 

 them in showing them their defects, and the means of correcting 

 them. 



56. The Blind Spot. Close the left eye and with the right look 

 intently at the cross in the following diagram (Fig. 73), holding the 

 book about fifteen inches in front of the eye. Both cross and circle 



Fig. 73. Diagram to demonstrate the existence of the blind spot. 



are seen. Gradually bring the book nearer the eye; at a certain 

 distance the circle disappears because its image falls upon the 

 entrance of the optic nerve. Bring the book still nearer; the circle 

 reappears. 



57. Field of Acute Vision. Look at a printed page without moving 

 the eyes and observe how few words can be seen distinctly. The 

 diameter of this field of distinct view will probably be found to be 

 about one and one-half inches. Wherever the image of an object 

 falls outside the yellow spot, it is seen indistinctly. 



58. Binocular Vision. Hold before each eye a blackened tube of 

 pasteboard. Two distinct fields will be seen on looking through the 

 tubes. Cause the tubes to converge at their free ends, and the two 

 fields will finally fuse into one. This position of the two tubes repre- 

 sents approximately the normal convergence of the two optical axes. 

 Converge the tubes still more ; the two fields reappear, but they are 

 crossed. Look through the tubes at near and at far objects. It will 

 be found that, in order to have a single field of vision, the tubes must 

 be converged more for the near objects. 



59. Close one eye, and, looking steadily ahead, note how much is 

 comprised in the field of view. On opening the eye the field is con- 

 siderably enlarged. 



60. Holding the forefinger six or eight inches in front of the nose, 

 look at a distant object, as a tree. The forefinger appears double. 

 Now accommodate the eyes for the finger; the tree appears double. 

 An explanation of this can be deduced from Ex. 58. 



61. Movements of the Eye. Close one eye, and, holding the finger 

 tip on the lid, feel the movements of that eye as the other eye looks 

 about in various directions. 



