ABNORMALITIES IN THE REFRACTION OF THE EYE 



853 



emitted by an object, are always projected upon the blind spot of one eye, this 

 defect is overcome in binocular vision by the fact that the corresponding rays in 

 the opposite eye are focalized outside this area and are therefore able to produce 

 a precise and complete impression in consciousness. 



Since the sensitiveness of the retina diminishes steadily from center to per- 

 iphery and also shows certain minor fluctuations in different persons, it cannot 

 surprise us to find that the visual field frequently possesses marked irregularities. 

 Furthermore, it must be evident that the luminosity and color of an object have 

 much to do with its size, because a white disc invariably yields a larger field than 



06 L 081 01-^ 



Fig. 453. — Perimeter Chart to Show the Field of Vision for a Riqht Eye When 



Kept in a Fixed Position. 

 J 

 one poorly illuminated or colored. Consequently, definite conclusions regarding 

 abnormalities of the visual field can only be drawn from a perimetric chart 

 which has been obtained under test conditions. 



CHAPTER LXXIII 

 ABNORMALITIES IN THE REFRACTION OF THE EYE 



Constant Optical Defects of the Eye. — In a perfect dioptric system 

 the media are absolutely transparent. This is not the case in the 

 human eye, because if a strong beam of light is thrown into its pupil, 

 the light is diffused by the different luminous points of its refractive 

 media. In fact, in many instances true opacities may be detected 

 which are dependent upon the presence of formed elements within the 



