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CONSCIOUS NERVOUS OPERATIONS 



glass or other biconvex lens can be arranged on a support, so as to focus 

 the rays of light admitted through the pupil upon a movable screen at 

 the back part of the box. A serviceable ground glass screen can be made 

 by rubbing a piece of ordinary window glass with emery powder and 

 water. The box is left open on one side to permit observation and 

 manipulation ; but while experimenting the observer will find it neces- 

 sary to exclude all light, except that which enters through the end of 

 the box. This can be done by covering the box and the head of the ob- 

 server with a black cloth. While with this apparatus one can illustrate 



Fig. 71. Apparatus for illustrating the optical properties of the eye. 



most of the characteristic optical features of the eye, yet obviously it 

 shows nothing regarding the functions of the aqueous and vitreous 

 humors. It must be borne in mind, also, that in the eye accommo- 

 dation for objects of different distances is brought about by changes 

 in the form of the lens ; here in the model by changes in the position 

 of the lens. Shortsightedness and longsightedness can be very easily 

 illustrated ; also the means of correcting them by spectacles. Astig- 

 matism can also be shown by holding in front of the artificial cornea 

 a bottle with sides of unequal curvature, filled with water. 



51. Accommodation. Hold up the forefinger six or eight inches 

 from the eye. Close one eye and look at the finger ; it appears dis- 

 tinct, while objects across the room seem blurred. Look at these 

 latter ; they become sharply outlined, but the outline of the finger 

 becomes indistinct. Notice that in accommodating for the near 

 object there is a feeling of effort. Cease looking at anything in par- 

 ticular, and allow the eyes to come to rest. They will be found to be 

 accommodated for distant vision. 



52. Ask a person to accommodate his eye for distant objects. Then 

 look at his eye from the side, while he adapts his vision to a near 



