178 



SENSORIAL FUNCTIONS. 



black and white ; and when a child, never could distin- 

 guish the cherries on the tree from the leaves, except by 

 their shape and size. The eyes of two of his brothers 

 were equally defective. 



A tailor at Plymouth, as described by Mr. Harvey of 

 the same place, regarded the seven prismatic colors as 

 consisting of only yellow and light blue. In other 

 respects he could only distinguish with certainty gray, 

 white, and yellow. This defect sometimes led him to 

 ludicrous mistakes in his business. Thus, on one occa- 

 sion he repaired a black silk garment with crimson, and 

 on another he patched the elbow of a blue coat, with a 

 piece of crimson cloth. 



A still more ludicrous case is given by Dr. Nichol, of 

 an officer in the British Navy, who purchased a blue 

 uniform coat and waistcoat, with red breeches to match. 

 Mr. Dugald Stewart was unable to distinguish any dif- 

 ference between the scarlet color of the Siberian crab 

 apple, and the leaves of the tree. 



Mr. Troughton, the celebrated optician, can distin- 

 guish with certainty only blue and yellow. 



No satisfactory solution has been given of the cause 

 of these defects. 



COMPARATIVE PHYSIOLOGY OF VISION. 



The lowest orders of animals, have no organs of vi- 

 sion which have ever been detected, and yet some of 

 them have been supposed to be in a slight degree sensi- 

 ble to the impression of light. Thus it is said, the Me- 

 dusa?, in a calm sea, are seen to rise towards the 

 surface, until coming within the full influence of the 

 sun's rays, they descend again before any part of their 

 bodies come in contact with the atmosphere. The 

 cause of the descent, and the reason why they never 

 expose their bodies above the water, has been supposed 

 to arise from the distinction they are able to make, 

 between the light near the surface and that of the deep 

 sea. It is most probable, however, that these animals 

 are guided by the pressure of the water, rather than by 

 the impression of light. 



