of Edinburgh, Session 1874-75. 
505 
The following Communications were read : — 
1. On the Diurnal Oscillations of the Barometer. 
By Alexander Buchan, M.A. 
2. The Phenomena of Single and Double Vision, as shown 
in the Stereoscope. By R. S. Wyld, Esq. 
When we direct the axes of the two eyes to any definite object, its 
different parts affect corresponding parts of each retina, and the 
object appears single. When we squint, or do not look direct at the 
object, its images affect non-corresponding parts of the two retinae, 
and the object appears double. The more widely the axes of the 
eyes are deflected from the object, the further asunder the double 
objects seem ; and the less the axes are deflected the less distant 
from each other the double objects appear. Thus, when we hold the 
finger in front of the eyes while we look at a distant candle or gas jet, 
the flame appears single and the finger double. When we turn the 
eyes to the finger it appears single and the flame appears double. 
The paper read to the Society in February 1871 was an attempt 
to prove that all the phenomena connected with single and double 
vision were explanable on the supposition that the nerve fibres of 
each retina decussate in a common cerebral sensorium, as for 
instance in the corpus guadrigeminum , which the optic nerves are 
known to enter ; and that as, owing to the fineness of the texture, 
anatomists had hitherto been unable to determine the ultimate 
arrangement of the fibres in the brain, we were justified in making 
this suggestion. 
Such a crossing of nerve fibres has in it nothing improbable, for 
there are many instances of such crossings, as for instance in the 
great and in the lesser commissures of the brain. There is also a 
similar crossing in the medulla oblongata of the motor nerves from 
the brain, before they descend the spinal cord ; and there is a similar 
crossing of the sensory nerves where they enter the spinal cord. 
The supposition then of a decussation of the fibres of the optic 
nerve within the brain is in analogy with what we know to be of 
frequent occurrence in the body. 
The facts which Mr Wyld now brings before the Society are these — 
