352 Proceedings of Royal Society of Edinburgh. [sess. 
Note on Normal Nystagmus. By Prof. Crum. Brown. 
(Read February 4, 1895.) 
When, with head fixed, we allow the eyes to wander over a fixed 
scene before us, we find that the eyes do not move continuously, but 
by jerks, so that what we see is really a series of separate pictures, 
each of which is at rest, the jerk from one position to the next 
being so rapid that we practically see nothing during the change of 
picture. That this is so can be shown if we have a bright light — 
say, an incandescent electric lamp — in the field of vision. After 
allowing the eyes to wander over the scene before us, we find, on 
closing the eyes, that we see a series of separate sharp secondary 
images of the bright light. Even when we make an effort to move 
the eyes continuously we find, by means of the secondary images, 
that we have not succeeded in doing so.* 
If, instead of keeping the head absolutely still, we move it, we 
find that in this case also the lines of glance — that is, the lines 
along which we look during the intervals between the jerks — remain 
fixed in reference to fixed external objects. It is obvious that 
during the interval between two jerks the muscles of the eyeballs 
must act so as exactly to compensate for the movement of the head, 
and this whatever be the axis about which the head is moving. 
Even when this axis is the line of vision, the compensating move- 
ment takes place, as may be shown by a simple experiment. 
Select an object upon which the eye is to be kept rigorously fixed 
during the movement of the head. This object should be about 15° 
or 20° distant from the bright light. Now, keeping the eye 
fixed on the mark, incline the head towards one shoulder, thus 
rotating it about a fore-and-aft axis, and then shut the eyes. We 
* There is a device, however, by means of which we can move our eyes con- 
tinuously so as to see, not a series of fixed pictures, but one moving pieture, and 
get, not a number of distinct and sharp after-images of the bright object, but a 
band composed like a continuous spectrum of an infinite number of images, 
each infinitely near its neighbours. We can obtain tins result if we have a 
moving object in the field, and keep our eyes constantly fixed on it. 
