1894 - 95 .] Prof. Crum Brown on Normal Nystagmus. 353 
shall see distinct sharp after-images of the light, showing that the 
eyes were, during the glances, fixed relatively to external objects, 
and therefore rotating relatively to the head about a fore-and-aft 
axis at the same angular rate as the head relatively to external 
fixed objects and in the opposite sense. All these compensating 
movements of the eyeballs relatively to the head have the effect of 
giving us a succession of fixed pictures of fixed things across which 
we see any really moving object move. We thus obtain that sense 
of the steadiness of the external world which is of great use to us 
in moving about in it. 
These compensatory movements of the eyes are successful only 
when the angular movement of the head is not too rapid, for there 
is a maximum rate beyond which the eyeball cannot go : if the 
head moves faster than that, the eye does indeed make an effort to 
compensate and give us steady pictures, but these efforts are only 
partially successful. We can wag our head faster than the eyeballs 
can move; and when we do so, we see the world wag in the 
opposite direction; but the really fixed external objects which we 
see wagging do not seem to describe nearly so large an angle as the 
head actually does : the movements of the eyes compensate to a 
certain extent, though not completely, the movement of the head. 
When we look for the cause of the phenomena we have been 
describing, we encounter a difficulty which is of the same kind as 
that which meets us in almost every question concerning our 
senses. We have been accustomed from our earliest infancy to use 
our senses, and our power of moving our eyes, our head, and our 
hands, so as to obtain, in ordinary cases, a consistent notion of what 
goes on around us ; and we cannot usually tell how much of our 
information has come to us through one channel, and how much 
through another ; but I think I shall be able to show that there 
are at least two causes of the normal nystagmus I have been de- 
scribing. Helmholtz, who has described most of these phenomena, 
and has shown how they satisfactorily explain some striking and 
interesting optical illusions, refers them exclusively to the effort to 
fix external objects. We do not see anything very well unless we 
look at it, and to look at it we must keep our eyes fixed on it for 
an appreciable, though it may be a very short, time. This effort of 
fixation satisfactorily explains the jerkings of the eyes when the 
3 / 4 / 95 . 
VOL. XX. 
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