June 20, 1895] 



NATURE 



185 



look at an objectless field of view, such as ihe clear, cloudless 

 sky, or, as was, I believe, first noticed by Dr. lireuer, when the 

 eyes are shut. By placing the fingers lightly over the closed 

 eyelids we can feel the motion of the prominent cornea. If, 

 with eyes shut and fingers so placed on the eyelids, we turn the 

 head or turn head and body round, we feel the eyes twitch. As 

 the head turns round the eyes retain for a little a fixed orientation 

 in respect to external fixed things, and then jerk so as to make 

 up for lost time, again pause, and again jerk, and soon. .So that 

 while the head turns uniformly, the eyes, which iintst, of course, 

 on the whole make one full turn, while the head makes one full 

 turn, do their rotation intermittently, being, so to speak, left 

 behind by the head, and then making up by a rapid jerk. 



Another proof that these compensatory movements, as they 

 may be called, of the eyeballs are not, or. at least, not wholly, 

 caused by the effort of looking at things, is afforded by ob- 

 serving what happens when the head is rotated about a fore and 

 aft axis, about an axis coinciding with a glance-line. If we keep 

 our eyes fixed on a particular point and rotate the head about 

 the line along which we look,' we still see things fixed, the world 

 does not seem to revolve about our fore anrl aft axis. Here also 

 we can show by means of secondary images that we see a series 

 of fixed j^ictures. 



If, with a bright object in the field of vision, we fix our eyes 

 and keep them fixed on a jioint, about 15° distant, from the 

 bright object (if we keep both eyes open, about as far from our 

 eyes as the bright object is, so as to avoid double vision), and 

 then rotate the head about a fore and aft axis through, say, 30° by 

 inclining the head towards one shoulder, and shut the eyes after 

 this performance, we see a number of sharji secondary images of 

 the bright object arranged upon an arc of a circle, the radius of 

 which is the angular distance of the bright object from the point 

 fixed. 



If I have rotated my head through about 30', I see about five 

 secondary images, so that what I call the atr^li' of rotatory 

 nystagmus is, in my case, about 6°. Here we have been looking 

 all the time at the same jioint, and it is not easy to su]ipose that 

 the very slight attention we pay to objects seen indirectly, or, as 

 we sometimes say, " with the tail of the eye," could lead to a 

 habit, so fixed that we cannot escape it, of moving the eyeballs 

 in the way described. 



I have said that the movement of the head, unless it is very 

 rapid, does not affect the fixedness of the glance-lines. Trans- 

 latory motion of our body may be so rapid, as in a railway train, 

 that the eyes cannot twitch so fast as to keep the glance-lines 

 fixed relatively to near fixed objects. 



The eyes do their best, they twitch but not enough, unless 

 the train is moving slowly, and near objects seem to fly back- 

 wards. We succeed with fixed objects at a greater distance 

 from us ; we can see them fixed, and all ol)jects between us and 

 .such visibly fixed objects are seen to move liackwards, fixed 

 things beyond tliem seem to mo\e forward with us. Of course 

 if, by keeping our attention on our carriage and its contents, our 

 glance-lines bect)me fixed in reference to these really moving 

 things, they seem fixed, and the whole world outside of the 

 carriage is seen to move in the direction opposite to that of our 

 real motion. It is also obvious that rotation of Ihe head, if it 

 is more rapid than the quickest possible rotation of the eyeball 

 in the head, must affect the position of a glance-line, for, in 

 order that the gl.ance-line may remain fixed, the eyeball must 

 rotate in reference to the head as fast in (me sense as the head 

 rotates in reference to external things in the other .sense ; but in 

 the case supposed, the eyeball cannot do so. We can try this 

 experiment without having recourse to mechanical means of 

 rotating our body and head, which, of course, we could do as 

 fast as we please, and a great deal faster than would be either 

 ]>leasant or safe. The most rapid rotation of our head which 

 we can produce by the direct action of our muscles is what is 

 known as wagging, that is, a rotation about a vertical axis upon 

 the joint between the first two vertebra;. In this way we can 

 give the head an angular velocity considerably greater than the 

 maximum angular velocity of the eyeball. When we do this as 

 fast as we can, we see that external things do not apjiear steady. 

 When we wag our head to the right we see the world wag to 

 the left, and -.'in- versa. Hut the external really fixed things 

 do not appear to us to describe nearly so large an angle as the 



1 If we lalce.l sufficiently disl,-im object as the tiling to be loclicd at, wc 

 may neglect the want of coincidence of tlie two glance-lines Ijeloiiging to ttic 

 two eyes, andj moreover, all that is here described is seen as well, though 

 not so conveniently with one eye shut. 



NO. 1338, VOL. 52] 



head really does, the eyes make an effort to compensate the 

 rotation of the head, an effort only partially successful, the angle 

 through which external things seem to move being the difference 

 between the actual angular rate of movement of the head, and 

 the maximum possible angular rate of movement of the eyeball 

 in its socket. This diflerence can best be observed and, indeed, 

 can be approximately measured by observing a distant light on 

 a dark night, while we wag the head. The point of light seems 

 drawn out into a horizontal line of light, the apparent length or 

 which is the angular diflerence in (piestion. As we can wag 

 our head much faster than we can nod it, the apparent length 01 

 the vertical line of light into which a bright point is drawn out 

 when we look at it and nod as rapidly as we can, is much less 

 than that of the horizontal line of light just spoken of; but I 

 find that I can, by nodiling, rotate my head about a right and 

 left axis a little faster than I can rotate my eyes aVjout the same 

 axis, so that the luminous point does appear drawn out into a 

 short vertical line. 



Such violent movements of the head occur sometimes in our 

 ordinary (not experimental) use of our eyes, but they are rare 

 and isolated, so that the disturbance of the fixedness of the 

 glance-lines which they cause does not really afi'ect our sense 01 

 the fixedness of the world. The illusion of the moving pier and 

 fixed steamer, which we have all also observed when there is a 

 train alongside that in which we happen to be, and we see 

 the moving train fixed and the fixed train moving, is cor- 

 rected by looking at the shore or the railway station. For a 

 moment these also seem to move, but our glance-lines almost 

 instantly become fixed in reference to these things which we 

 know are fixed, and it is then difficult to recall the illusion. 

 Another similar case is that of the moon and the clouds. We 

 sometimes see the moon moving and the clouds fixed, sometimes 

 the clouds moving and the moon fixed, as our glance-lines are 

 fixed relatively to the clouds or to the moon, and a little 

 practice enables us to change from the one sensation to the other 

 at will. 



W'hat has been said seems to show that our immediate sense 

 that the earth and what we call fixed objects on it are fixed is a 

 consequence of the way in which we move our eyes, and, in 

 particular, of the way in which, by a suitable movement of the 

 eyeballs, we involuntarily and unconsciously comjiensale move- 

 ments of the head, voluntary or involuntary, conscious or 

 unconscious.' 



That such an immediate sense of the fixedness of external 

 fixed things is of great use to us in moving about among them is 

 plainly shown w hen we observe the trouble which a drunken man, 

 who has lost this sense, has in guiding himself. 



I now turn to the qviestion. What is the cause of this ]irompt 

 and wonderfiilly accurate compensatory movement of the 

 eyeballs ? 



There are tliree sources from which we can obtain information 

 leading to an answer. ( I ) Kxiieriments on ourselves, (2) anato- 

 mical observations and measurements, and (3) observations of 

 the effects of injuries to the labyrinth of the internal ear. 



I shall consider these in their order. 



By experiments on ourselves I mean the study of the effect on 

 the motion of the eyes and on our sense of the fixedness of 

 external things, of movements of our head (in this case, always 

 along w ith the rest of our body) which we do not make, as a rule, 

 for any other purjiose. 



I have already staled that if we shut our eyes, place our fingers 

 on the eyelids, and turn roundabout a vertical axis, we feet with 

 our fingers the jerking motion of the eyeballs. If instead of 

 turning once round, we turn round several times, still better if 

 we seat our.selves on a turning-table and get some one else to turn 

 it and us round at a uniform rate, we find that the jerks become 

 less and less frequent, and after two or three turns cease alto- 

 gether. .Another thing which we observe is, that although the 

 turn-table is being turned round at a perfectly uniform rate, we 

 feel the rotation becoming slower and slower, and when the 

 jerks of the eyeballs have quite ceased we feel ourselves at rest, 

 and have no sens.alion of rotation. Let us for convenience call 

 the sense in which the rotation is still going on positive. This 

 uniform juisitive rotation has become to usim]>erce]itible (as King 

 as we keeji our head in the same ]iosition in respect to the 

 vertical), and is what we may call a new zero of rotation. If the 

 rale of rotation is now increased, we feel this increase as a positive 

 rotation ; if it is diminished, we feel the diminution .as a negative 



1 I need hardly repeat that, by movements of the he.ld, I mean movements 

 of the head wliether accompanied or not by movj nents of the body. 



