INSENSIBILITY TO COLOURS. 113 



objects double is too common to be noticed as remarkable ; 

 and though it may take place with only one eye, yet, as 

 it generally arises from a transient inability to direct the 

 axes of both eyes to the same point, it excites little notice. 

 That state of the eye, however, in which we lose sight 

 of half of every object at which we look, is more alarming 

 and more likely to be ascribed to the disappearance of 

 part of the object than to a defect of sight. Dr. Wollaston, 

 who experienced this defect twice, informs us that, after 

 taking violent exercise, he " suddenly found that he could 

 see but half of a man whom he met, and that, on attempt- 

 ing 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 reappear, while the other would 

 disappear in his turn. The circumstances under which 

 these evanescences would take place could not be supposed 

 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 insensi- 

 bility of the eye to particular colours. This defect is not 



