796 



THE EYE AND VISION 



[GIL LVI. 



for different colours ; thus a blue object is seen to be blue over a 

 wider field than a red, and a red over a wider field than a green 

 object. 



60 



60 



120 



140 



140 



160 



160 



180 



Fio. 592. Perimeter chart for the right eye. 



In disease of the optic nerve, contraction of the field of vision 

 for white and coloured objects is found. This is often seen before 

 any change in the optic nerve is discoverable by the ophthalmoscope. 



The yellow spot of one's own eye can be rendered evident by 

 what is called Clerk-Maxwell's experiment : On looking through a 

 solution of chrome-alum in a bottle with parallel sides, an oval 

 purplish spot is seen in the green colour of the alum. This is due 

 to the pigment of the yellow spot. 



Colour Sensations. 



Colours may differ (1) in hue, for instance, blue, red, yellow ; (2) 

 in saturation, for instance, pale green and full green ; this depends 

 upon the degree of admixture with white light ; and (3) in intensity, 



