of Edinburgh^ Session 1864-65. 373 
velopment, and sustained life of marine vegetable and animal 
organisms. 
3. On Hemiopsy, or Half Vision. By Sir David Brewster, ^ 
K.H., F.E.S. 
After describing the phenomena of hemiopsy, as observed by Dr 
Wollaston, M. Arago, and Mr Tyrrell, the author remarked that no 
attempt was made by these writers to ascertain the optical con- 
dition of the eye when it is said to be half blind, or to determine 
the locality and immediate cause of the complaint. Having ex- 
perienced several attacks of hemiopsy, unaccompanied with any 
affections of the head or stomach, the author found that there was 
no insensibility to light, but merely an insensibility to the lines 
and shades of the object which disappeared. This insensibility 
commenced in both eyes, a little to the left of the foramen centrale^ 
and extended itself irregularly to the margin of the retina on the 
left side. The parts of an object, or the letters of a word which 
disappear, are as bright as the ground around them, and are white 
if the ground is white, and always of the colour of the ground, so 
that the light of the ground has irradiated into the dark lines or 
shades of the picture on the retina, a phenomenon which can be 
produced in a sound eye by oblique vision.* This species of irra- 
diation, however, is merely a local and temporary paralysis of the 
retina by the continued action of light upon the same part of it ; 
but in hemiopsy, the irradiation is produced by the pressure of the 
blood-vessels, which may arise from various causes, — from the mere 
fatigue of the eye after long reading or exposure to bright light, 
or from affections of the head or stomach. That this pressure of 
the blood-vessels was the cause of the hemiopsy studied by the 
author, was proved by his going accidentally into a dark room while 
under its influence, when he was surprised to observe that all the 
parts of the retina which were affected were slightly luminous— an 
effect invariably produced by pressure upon that membrane. 
* Letters on Natural Magic. Letter II. p. 13. 
