SIGHT. 171 



What is color-blindness? 



An inability to perceive some colors. The colors which are 

 usually mistaken are green and red. Frequently it is found that 

 a distinction cannot be made between these colors. This is some- 

 times known as Daltonism. 



What is the cause of color-blindness ? 



It is probably due to the absence of the rods and cones which 

 are capable of responding to the stimulus of rays of a certain wave- 

 length. 



Is this an important defect ? 



Yes. In marine and land locomotion red and green signals are 

 used to indicate opposite conditions, and the failure to distinguish 

 them has frequently been the cause of serious accidents. 



How is color-blindness tested? 



By laying a number of skeins of yarn of various colors in a 

 heap, and requiring the person to be tested to select all resembling 

 a certain skein from the heap. 



Do the eyes act both at once, or. do we see with one at a time ? 

 We use both in ordinary vision at the same time. 



What is diplopia ? 



It is the condition which results from a want of harmony in the 

 eyes, so that the image of each eye is perceived separately ; that 

 is, one sees double. 



What are the common causes of diplopia ? 



Paralysis or spasm in one of the lateral straight muscles, which 

 does not allow the eye to be turned in harmony with the other. 

 If the eyes are turned so that the axes of vision are separated, the 

 condition is known as external strabismus or squint; if the axes 

 are crossed, it is called internal strabismus or cross-eye. 



What benefits result from binocular vision? 



The ideas of form and distance are much more correctly judged 

 by the perceptive faculties. This is due to the fact that the object 

 viewed is seen beyond its equator, so to speak, by each eye, and 

 the combined image is therefore less flat in appearance than in 

 monocular vision. 



