157 
of Edinburgh, Session 1869-70. 
with the insensitive spot where the optic nerve joins the retina. I 
may point out here, however, an experiment that shows the general 
peculiarity, and also the excess of change that takes place when 
the object is on the nasal side compared with the other. Against 
a dark-coloured wall hold up, at arm’s-length, an orange-coloured 
object of three or four inches in diameter. We suppose it held by 
the right hand ; then turning the face rather towards it, look at a 
point in the wall eighteen or twenty inches to the left of the object ; 
and now closing the eyes alternately, it will be observed that, when 
the right eye is open, the object will appear of nearly its full orange 
colour, but when the right eye is closed and the left opened, the 
object will assume a pale, sickly, yellow tint ; and if the point in 
the wall be taken further from the object, the colour seen by the 
left eye will approach nearer to white. To cause the same amount 
of change to the right eye, the obliquity must be very much greater. 
Another mode of conducting the experiment, as depending upon 
the contrast of effect upon the two sides of the eye, is this : Choose 
two objects of the same colour, place these two or three inches 
above or below a mark on the wall, close one eye, and with the 
hands withdraw the objects equally away on either side from the 
central position, the eye being rivetted to the mark on the wall ; 
it will then be noticed that, relatively, the object on the nasal side 
of the observing eye undergoes a rapid change of tint or colour. 
But, it may be repeated, the most satisfactory mode of examining 
the changes is to use one eye and observe with the coloured object 
on the nasal side of it, the eye being held steadily upon a mark, 
which may or may not be of the same colour as the object. Observed 
in this way, the following changes will be presented : — 
First. The colours lose more or less their chromatic intensity, 
and approach nearer to white or black, according as they are placed 
upon a dark or light ground. But extreme red is especially marked 
as losing illuminative power, as well as chromatic character. Ultra- 
marine blue, on the contrary, appears to lose very little by oblique 
vision ; it assumes a lighter blue hue. 
Second. The colours undergo a change of chromatic character. 
a. Brilliant scarlet , painted with biniodide of mercury and 
gum arabic. — This, when placed on a dark ground, and 
observed at an obliquity of about 30° on the nasal side, 
VOL. VII. x 
