STUDIES IN SPECIAL SENSE PHYSIOLOGY :'.'.:{ 



must next consider some types of vision which, although abnormal, 

 throw light upon the normal mechanism. These are the commoner 

 forms of congenital partial colour-blindness. 



The existence of abnormal colour perception in certain people 

 has been recognised for more than two centuries ( n ) ; j but John 

 Dalton, the great chemist, was the first who attracted much 

 attention to the subject ( 12 ). 



Goethe in his Farbenlehre, which appeared in 1812, gives the 

 following description : ( 13 ) 



" We will here first advert to a very remarkable state in which 

 the vision of many persons is found to be. As it presents a 

 deviation from the ordinary mode of seeing colours, it might fairly 

 be classed under morbid impressions ; but as it is consistent with 

 itself, as it often occurs, may extend to many members of a family 

 and probably does not admit of cure, we may consider it as bor- 

 dering only on the nosological cases and therefore place it first. 



" I was acquainted with two individuals not more than twenty 

 years of age, who were thus affected. . . . They agreed with the 

 rest of the world in denominating white, black, and grey in the 

 usual manner. . . . They appeared to see yellow, red-yellow, and 

 yellow-red like others. . . . But now a striking difference presented 

 itself. If the carmine were passed thinly over the white saucer, 

 they would compare the light colour thus produced to the colour of 

 the sky and call it blue. If -a rose were shown them beside it, 

 they would in like manner call it blue ; and in all the trials that 

 were made, it appeared that they could not distinguish light blue 

 from rose colour. 



" They confounded rose colour, blue, and violet on all occa- 

 sions ; these colours only appeared to them to be distinguished 

 from each other by delicate shades of lighter, darker, intenser, or 

 fainter appearance. 



" Again they could not distinguish green from dark orange, 

 nor, more especially, from a red-brown. 



" If any one accidentally conversing with these individuals, 

 happened to question them about surrounding objects, their answers 

 occasioned the greatest perplexity, and the interrogator began to 

 fancy his own wits were out of order. With some method we 

 may, however, approach to a nearer knowledge of the law of this 

 deviation from the general law. 



" These persons, as may be gathered from what has been stated, 



