16 SIR DAVID BREWSTER ON HEMIOPSY, OR HALF-VISION. 



obscurity takes place in different portions of the retina, and varies in its extent at 

 different times. 



Having myself experienced several attacks of hemiopsy, I have been enabled 

 to ascertain the optical condition of the retina when under its influence, and to 

 determine the extent of the affection, and its immediate cause. 



In reading the different cases of hemiops}-, we are led to infer that there is 

 vision in one-half of the retina, and blindness in the other. But this is not the 

 case. The blindness, or insensibility to distinct impressions, exists chiefly in a 

 small portion of the retina to the right or left hand of the foramen cent rale, 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 paralysed 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 dark- 

 ness 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 fora- 

 men, 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 occupied 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 paralysed 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 irra- 

 diation of the light into the space occupied with the letters, or the objects which 

 disappear, though a phenomenon of the same kind as that which takes place in 



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



