INSENSIBILITY TO COLOURS. 29 



after taking violent exercise, he " suddenly found 

 that he could see but half of a man whom he 

 met, and that on attempting to read the name of 

 JOHNSON over a door, he saw only SON, 



the commencement of the name being wholly 

 obliterated from his view." In this instance, the 

 part of the object which disappeared was towards 

 his left ; but on a second occurrence of the same 

 affection, the part which disappeared was towards 

 his right. There are many occasions on which 

 this defect of the eye might alarm the person 

 who witnessed it for the first time. At certain 

 distances from the eye one of two persons would 

 necessarily disappear ; and by a slight change of 

 position either in the observer or the person 

 observed, the person that vanished would re- 

 appear, while the other would disappear in his 

 turn. The circumstances under which these 

 evanescences would take place could not be sup- 

 posed to occur to an ordinary observer, even if 

 he should be aware that the cause had its origin 

 in himself. When a phenomenon so strange is 

 seen by a person in perfect health, as it generally 

 is, and who has never had occasion to distrust 

 the testimony of his senses, -he can scarcely refer 

 it to any other cause than a supernatural one. 



Among the affections of the eye which not 

 only deceive the person who is subject to them, 

 but those also who witness their operation, may 

 be enumerated the insensibility of the eye to 

 particular colours. This defect is not accom- 

 panied with any imperfection of vision, or con- 

 nected with any disease either of a local or a 

 general nature, and it has hitherto been observed 

 in persons who possess a strong and a sharp 

 sight. Mr. Huddart has described the case of 



