32& SIR DAVID BREWSTER ON A 



zig-zag lines proceeding, as in Fig. 2, from the foramen outwards, and not in a 



circular arch, as shown in Fig 1. The " general obscuration," mentioned by 



Mr Aiky, shows that the luminous impression on the 



affected parts is not so strong in his case as in mine, and 



that the retina is still sensible to light derived from the 



surrounding parts by irradiation. The severity of the 



affection in Mr Airy's case is remarkable. In mine the 



attack is little more than disagreeable, and I have never 



experienced the slightest effect either upon the speech 



or the memory. I have given this brief abstract of Mr 



Amy's interesting paper from the relation of hemiopsy 



to the permanent affection of the retina which I am 



about to describe. 



When without the hope of obtaining any precise information respecting the 

 irradiation into the parts of the retina affected with hemiopsy, an accidental 

 observation revealed to me the disagreeable fact that a considerable portion of 

 the retina of my right eye was absolutely blind, or insensible to visual impres- 

 sions ; and I have thus been enabled, from the extent and permanence of the 

 affection, to make whatever observations were necessary to ascertain the true 

 character of the phenomenon. 



The portion of the retina thus affected with what may be called local 

 amaurosis is situated, in the field of vision, about 15° from the foramen, in a line 

 to the left inclined 45° to the horizon. Its angular magnitude is about G° in its 

 greatest breadth, which corresponds to a space about the twenty-eighth of an 

 inch on the retina. 



When the image of a bright object covers the whole, or any part of this spot, 

 it is invisible. If the image is the flame of a candle, or of the moon, or of the 

 sun near the horizon, it is wholly invisible. The eye is therefore at this part of 

 it absolutely insensible to light falling upon it from without. If we now direct 

 the eye to the sky, to the white ceiling of a room, or to any extended white sur- 

 face, no dark spot, even of the slightest shade, is seen in the field of vision. The 

 portion of the retina, therefore, insensible to light incident upon it directly, or 

 from without, has been illuminated by irradiation from the surrounding parts. 

 But for this wise provision, an eye affected with local amaurosis would carry 

 about with it a black spot, disfiguring the aspects of nature, and ever reminding 

 the patient of his misfortune. 



How long this condition of my retina has existed, I cannot discover. It may 

 have existed for half a century, or more ; and, but for a casual observation, its 

 existence might never have been discovered. Whether it came on gradually, or 

 was produced in some of the experiments in which the eye was exposed to the 

 light of the sun, I have no means of ascertaining. If from the first of these 



