992 A MANUAL OF PHYSIOLOGY 



(a pin or the point of a pencil) towards the other eye till it becomes 

 blurred. Interpose between the object and the eye a card per- 

 forated by a small hole. The object becomes more distinct owing 

 to the cutting off of the peripheral rays (p. 912). 



8. Chromatic Aberration. Look at Fig. 391 (p. 913) from a dis- 

 tance too small for perfect accommodation, and verify the facts 

 given in the description of the figure. 



9- Measurement of the Extent of the Field of Vision. Use the 

 perimeter shown in Fig. 415 (p. 946). 



(i) For White Light. Fix in the holder, Ob, on the graduated arc, 

 a small piece of white paper, and put one of the charts supplied 

 with the instrument at the back of the wheel which revolves with 

 the arc. The observations can be recorded on this chart. The 

 patient rests his chin on K and adjusts one eye against O. This 

 eye is kept fixed on the mark at / during the whole period of observa- 

 tion, and the other eye is covered. The arc is placed in a definite 

 position, and the white object gradually moved from the end of 

 the arc until the person announces that he can just see it. The 



FIG. 441. MAP OF BLIND SPOT (REDUCED BY ONE-HALF). 

 Right eye. Distance of eye from paper, 12 inches. 



angle at which'this occurs is read off and recorded on the chart. 

 The aic is then rotated into a new position and the observation 

 repeated. A line is drawn through all the points thus obtained, 

 and this constitutes the boundary of the field of vision (Fig. 440). 



If the position of each point is inserted on the chart, a point above 

 the horizontal plane passing through the visual axis being placed 

 below it, and a point to the right of the vertical plane being moved 

 to the left, we obtain a map of the sensitive portion of the retina. 

 Usually perimeters are arranged to do this automatically. 



(2) Repeat the mapping of the field, using coloured papers (red, 

 green, and blue) instead of white. 



10. Mapping the Blind Spot. Make a black cross on a piece of 

 white paper attached to the wall, the centre of the cross being at the 

 height of the eye in the erect position. Stand about 12 inches from 

 the wall, the chin supported on a projecting piece of wood. Fix the 

 centre of the cross with one eye, the other being closed, and move 

 over the paper a pencil covered, except at the point, with white 

 paper, until the point just disappears. Make a mark on the paper 

 at this point, and repeat the observation for all diameters of the 



