PECULIAR DEFECTS OF VISION. 



the cases in which sight has been suddenly restored to adults blind 

 from their birth. In these cases, the first impression produced by 

 vision is that the objects seen are in immediate contact with the 

 eye. It is not until the hand is stretched forth to ascertain the 

 absence of the objects seen from the space before the eye that this 

 optical fallacy is dissipated. 



The eye which has recently gained the power of vision at first 

 cannot distinguish one colour from another, and it is not until 

 time has been given for experience, that either colour or outline 

 is perceived. 



96. Besides that imperfection incident to the organs of sight 

 arising from the excess or deficiency of their refractive powers, 

 there is another class which appear to depend upon the quality of 

 the humours through which the light proceeding from visible 

 objects passes before attaining the retina. It is evident that if 

 these humours be not absolutely transparent and colourless, the 

 image on the retina, though it may correspond in form and outline 

 with the object, will not correspond in colour ; for if the humours 

 be not colourless, some constituents of the light proceeding from 

 the object will be intercepted before reaching the retina, and the 

 picture on the retina will accordingly be deprived of the colours 

 thus intercepted. If, for example, the humours of the eye were 

 so constituted as to intercept all the red and orange rays of white 

 light, white paper, or any other white object, such as the sun, 

 for example, would appear of a bluish-green colour ; and if, on 

 the other hand, the humours were so constituted as to intercept 

 the blues and violets of white light, all white objects would ap- 

 pear to have a reddish hue. Such defects in the humours of the 

 eye are fortunately rare, but not unprecedented. 



97. Sir David Brewster, who has curiously examined and col- 

 lected together cases of this kind, gives the following examples 

 of these defects : 



A singular affection of the retina in reference to colour is shown 

 in the inability of some eyes to distinguish certain colours of the 

 spectrum. The persons who experience this defect have their 

 eyes generally in a sound state, and are capable of performing all 

 the most delicate functions of vision. Mr. Harris, a shoemaker 

 at Allonby, was unable from his infancy to distinguish the cher- 

 ries of a cherry-tree from its leaves, in so far as colour was 

 concerned. Two of his brothers were equally defective in this 

 respect, and always mistook orange for grass-green, and light 

 green for yellow. Harris himself could only distinguish black 

 and white. Mr. Scott, who describes his own case in the Phi- 

 losophical Transactions, mistook pink for a pale blue, and a full 

 red for a full green. 



95 



