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III. — On Hemiopsy, or Half- Vision. By Sir David Brewster, K.H., F.R.S. 



(Read 20th February 1865.) 



The affection of Half-vision, or Half-blindness as it has been called, was first 

 distinctly described by Dr Wollaston, in a paper " On Semidecussation of the 

 Optic Nerves, 1 ' published in the Philosophical Transactions for 1824. " It is now 

 more than twenty years," he says, " since I was first affected with this peculiar 

 state of vision, in consequence of violent exercise I had taken for two or three 

 hours before. I suddenly found that I could see but half the face of a man whom 

 I met, and it was the same with every object I looked at. In attempting to read 

 the name Johnson over a door, I saw only son, the commencement of the name 

 being wholly obliterated from my view. In this instance, the loss of sight was 

 towards my left, and was the same, whether I looked with my right eye or 

 my left. This blindness was not so complete as to amount to absolute black- 

 ness, but was a shaded darkness, without definite outline. The complaint 

 lasted only about a quarter of an hour." In 1822, Dr Wollaston had another 

 attack of hemiopsy, with this difference, that the blindness was to the right of 

 the centre of vision, and he has referred to three other cases among his friends ; 

 but in these, the affection was accompanied with headache and indigestion. 



In republishing Dr Wollaston's paper in the "Annates de Chimieet Physique"* 

 M. Arago says, that he knows four cases of hemiopsy, and that he himself had 

 experienced three attacks of it, followed by headache above the right eye. 



In the " Cyclopaedia of Practical Surgery," published in 1841, Mr Tyrrell 

 describes Hemiopsy as " Functional amaurosis from general disturbance." He 

 informs us that " he has experienced this form of amaurosis several times," and 

 that he has been consulted by several fellow-sufferers of both sexes. In all these 

 cases the affection was attended with severe headache, giddiness, and gastric irri- 

 tation, sometimes preceding, and sometimes following, the attack. 



In the accounts which have been given of these different cases of hemiopsy, 

 no attempt has been made to ascertain the optical condition of the eye when it is 

 said to be half-blind, or to determine the locality and immediate cause of the 

 complaint. Dr Wollaston describes the blindness as a shaded darkness without 

 definite outline. M. Arago says nothing about darkness ; and the insensibility of 

 the retina, of which he speaks, must mean its insensibility to visual and not to 

 luminous impressions. Mr Tyrrell, on the other hand, simply states, that the 



* 1824, vol. xxvii. p. 109. 

 VOL. XXIV. PART I. E 



