174 SENSORIAL FUNCTIONS. 



527. A shoemaker, named Harris, could only distinguish 

 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. 



528. 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 Hue. In other re- 

 spects he could only distinguish with certainty, gray, white 

 and yellow. This defect sometimes led him to ludicrous 

 mistakes in his business. Thus, on one occasion, he 

 repaired a black silk garment with crimson, and on another, 

 he patched the elbow of a blue coat, with a piece of crim- 

 son cloth. 



529. 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 differ- 

 ence between the scarlet color of the Siberian crab-apple 

 and the leaves of the tree. 



530. 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. 



531. The lowest orders of animals, have no organs ot 

 vision 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- 

 dusaB, in a calm sea, are seen to rise toward the surface, 

 until coming within the full influence of the sun's rays, 

 they descend again before any part of their bodies 

 comes 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. 



