Sir David Brewster on Hemiopsy, or Half- Vision. 505 



that there is vision in one-half of the retina, and blindness in 

 the other. But this is not the case. The blindness, or insen- 

 sibility to distinct impressions, exists chiefly in a small portion 

 of the retina to the right or left hand of the foramen centrale t 

 and extends itself irregularly to other parts of the retina on the 

 same side, in the neighbourhood of which the vision is uninjured. 

 In some cases the upper half of the object is invisible, the part 

 of the retina paralyzed being a little below the foramen centrale. 

 On some occasions, in absolute darkness, when a faint glow of 

 light was produced by some uniform pressure upon the whole 

 of the retina, I have observed a great number of black spots, 

 corresponding to parts of the retina upon which no pressure was 

 exerted. 



In the case of ordinary hemiopsy, as observed by myself, there 

 is neither darkness nor obscurity, the portion of the paper from 

 which the letters disappear being as bright as those upon which 

 they are seen. Now this is a remarkable condition of the 

 retina. While it is sensible to luminous impressions, it is in- 

 sensible to the lines and shades of the pictures which it receives 

 of external objects; or, in other words, the retina is in certain 

 parts of it in such a state that the light which falls upon it is 

 irradiated, or passes into the dark lines or shades of the pictures 

 upon it, and obliterates them. This irradiation exists to a small 

 degree, even when the vision is perfect at the foramen centrale, 

 and it may be produced artificially in a sound eye, on parts of 

 the retina remote from the foramen, and as completely, though 

 temporarily, as in hemiopsy. In order to prove this, we have 

 only to look obliquely at a narrow strip of paper placed upon a 

 green cloth — that is, to fix the eye upon a point a little distant 

 from the strip of paper. After a short time the strip of paper 

 will disappear partially or wholly, and the space which it occu- 

 pied will be green, or the colour of the ground upon which it 

 is laid*. 



This temporary insensibility of the retina in the part of it 

 covered by the picture of the strip of paper, or its inability to 

 maintain constant vision of it, can arise only from its being 

 paralyzed by the continued action of light, — an effect not likely 

 to be produced, and never observed, in the ordinary use of 

 the eye. 



The insensibility of the retina in cases of hemiopsy, and the 

 consequent irradiation of the light into the space occupied with 

 the letters, or the objects which disappear, though a pheno- 

 menon of the same kind as that which takes place in oblique 

 vision, has yet a very different origin. The parts which are in 

 these cases affected extend irregularly from the foramen centrale 

 * Letters on Natural Magic. Lett. II. p. 13. 



Phil Mag. S. 4. No. 199. SuppL Vol. 29. 2 L 



