THE SENSE OF VISION 



623 



animals which have acute daylight vision e.g., birds, such as the 

 pigeon and chicken but which see imperfectly in feeble or artificial 

 light. 



Further, m cases of total colour-blindness (achromatopsia), the 

 spectrum is seen merely as a band of light of varying intensity, the 

 greatest brightness being in the regions outside the fovea, while in 

 many cases a blind spot (scotoma) is found in a position corresponding 

 to the fovea. In such cases there is good vision in twilight, but in 

 daylight a marked diminution in the acuity of vision, a fear of strong 

 light (photophobia), bad fixation leading to nystagmus (quick side- 

 to-side movements of the eyes). 



In cases of nicotine-poisoning, with visual trouble (tobacco ambly- 

 opia), there is a loss of acuity of vision and deranged colour sensation. 

 It is the region of the fovea which is affected. 



In cases of " night-blindness " (hemeralopia or nyctalopia) there 

 is, in comparison with the normal colour sense, a shortening of the 

 violet end of the spectrum, and an impaired responsiveness to light 

 or short wave-lengths. For this reason, a person suffering from this 

 condition is unable to see well in twilight or artificial light, and is 

 said to be night-blind. This condition is inherited, transmitted by 

 the females, but present only in the males of a family. 



The Perception of Colour. When white light is passed through a 

 prism, it is broken up into a number of colours, owing to the greater 



FIG. 354. WHEEL FOR MIXING COLOUKS. 



refrangibility of some rays than others. To most people, the spectrum 

 is made up of six distinct colours: Red, orange, yellow, green, blue, 

 violet. Normal individuals may therefore be said to be " hexachromic." 

 A few people, however, can, like Newton, see a seventh colour indigo 

 between the blue and the violet. They are " heptachromic." 

 White light is therefore made up of a fusion of these colours. This 

 can be shown by passing the colours through 'a second prism, when 

 they are recombined to form " white " light. 



It is not necessary, however, that all the colours be fused to give 



