Nor did Hans possess anything like that high degree of visual acuity which had been attributed to him. He was supposed to be able to read easily at a distance small, almost illegible script, which we ourselves could decipher only with the greatest difficulty close at hand. It was also supposed that he could distinguish ten-and fifty-pfennig pieces whose faces had become worn beyond recognition for us. None of these accomplishments have stood the test. We have no reason to believe that Hans can see the objects about him more clearly than other horses, regarding whom one usually assumes that they receive only vague visual impressions. Horses do not as a rule seem to be near-sighted as is often asserted by the layman, but rather somewhat far-sighted, or if we may believe Riegel,[37] who tested some six hundred horses, they probably have normal vision. But we are told that many horses—and according to some authors all—have an innate imperfection which detracts considerably from the clarity of vision. This imperfection consists in an irregular formation of the sclerotic coat and of the lens of the eye.[38] The two organs do not have the same refraction in all parts. As a result, objective points are not imaged as points upon the retina. (Hence the name: astigmatism, i. e., "without points", for this disorder.) The retinal image of the object is not only vague, but also distorted.[X]
Many will doubt whether with such imperfect images an animal can react to directives so minute, as we have asserted to be true in the case of Hans. In considering this question we must distinguish between the directives for pointing out colors and the directives for tapping and for head movements on the part of the horse. In pointing out and bringing forth pieces of colored cloth there is involved the perception of an object at rest, viz.: the direction of the questioner who is standing quietly; whereas in the case of responses by means of tapping the stimulus is the horse's perception of the questioner's movements. Now, the construction of the horse's eye, as described above, is not favorable for the perception of objects (so-called acuity of vision). This may partly account for the slight success of the horse in those tests in which he was required to select a piece of cloth of a designated color, in so far as these commands were not accompanied by calls or exhortations. Where human observers averaged eighty per cent correct responses ([page 135]), Hans, under similar conditions was successful in only one-third of the tests. In his errors he was also wider of the mark than were the human observers ([page 82]). The object perceived, to be sure, is a large one, viz.: the questioner, and he at close range. We must therefore consider more specifically what are the determining factors that make for success or failure of the response. First of all, the innocent questioner very often did not designate the direction with sufficient clearness. Furthermore, Hans presumably was not able to discriminate sufficiently between the direction of the experimenter's eye and that of his head, which two directions did not always coincide. Finally the horse's attention was often diverted, while he was running toward the piece indicated, by the other pieces lying to the right and to the left, and for this reason the addition of a single piece to the otherwise unchanged row of five pieces tended to decrease greatly the chances of success.
The case is different with the perception of the directive signs for tapping, for nodding and shaking the head, etc., all of which require the perception of movements. This is not necessarily more difficult on account of the imperfect constitution of the tissues that serve for the refraction of light. Some authors even aver that this facilitates the perception of moving objects. This view was first advanced by the excellent ophthalmologist, R. Berlin[39] of Stuttgart. In arriving at this view he was guided by the following considerations. The peculiar form of astigmatism of the lens of the horse's eye, which Berlin has described as "butzenscheibenförmig",[Y] because it appears in the form of a series of glossy concentric circles around the lens nucleus, has the property of enlarging the pathway (and with it the rapidity) of moving retinal images. If we take a speculum by means of which a view may be had of the interior of the eye, and fixate a definite point on the retina of the horse, and then make a slight movement of the head horizontally, we find that the point fixated moves—apparently at least—toward the border of the pupil. In a normally constructed eye this seeming movement will be in a straight line, while in the eye of the horse, (according to Berlin), its path is curved, and therefore longer. Berlin believes that the same thing which here occurs in the case of this merely apparent movement, must also happen when an external moving object is imaged on the horse's retina. Its pathway, too, will be curved, and therefore longer, so that if the head of Mr. von Osten moves past the animal's eye, then the image on the horse's retina will take a longer, more circuitous route than it would if the eye were not astigmatic. We cannot, however, immediately conclude from the fact that an objective movement is imaged as being greater in extent on the retina, that it will therefore be more readily perceived by much less that it will appear greater to, the horse, than would be the case if the lens were normally constructed. The visual percept is not immediately dependent upon the retinal processes, for between the two are interpolated complex, inaccessible nervous processes. Still, Berlin believes that he is justified in drawing this conclusion from a number of relevant considerations. Accepting it, he believes that it would be possible for the horse to perceive movements, that for the human eye, which is not subject to this form of astigmatism, would lie below the threshold.
This theory, the simplicity of which certainly must make a strong appeal, has been adopted by a number of well-known investigators (Schleich[40], Königshöfer[41]). If we also could accept it, then Hans's phenomenal power of perceiving the movements of objects would be explained. But doubts arise which restrain us. Even if we were to accept Berlin's view in general, we should still come upon the following difficulties. In the first place, it is questionable whether the peculiar form of astigmatism mentioned is indeed as common as he supposes.[Z] The references in the literature are exceedingly meager on this point. In order to make a few tests at least, I undertook to examine nine horses with the aid of Dr. R. Simon, oculist, to whom I am greatly beholden for the assistance given in these and other tests to be mentioned presently. In not one of the nine cases did we discover anything like the curved deflection which is supposed to be the sign of the form of astigmatism in question. But in order to test objectively whether Berlin's assumption were justified, we examined in the laboratory fresh specimens taken from two horses. The eyes were fastened in a frame in what corresponded to their normal position. Their posterior spherical wall (i. e., their respective retinal surface) was replaced by a piece of ground glass. On a spherical surface linear movements of a point of light are always imaged as curves, no matter what the shape of the lens forming the image may be. (For a more detailed statement see [page 170, at close of note].) Since, however, our investigation had to do only with those curves which were due to the qualities peculiar to the lens, we had to replace the spherical by a plane projection surface. In front of the eye thus modified a strong light was placed at such a distance that the image of it, produced on the improvised back of the eye by the cornea and the lens, was a sharply defined point of light. Now, when the source of light was moved, the point of light would also move on the glass plate. Sitting at some distance behind the eye, we observed the movements of this point through a telescope. Thus we became witnesses of what happens upon the horse's retina when a moving object passes in front of his eye. Although we saw the point of light move through relatively long distances both horizontally and vertically, no sort of deflection in its pathway could be noted. Berlin's exposition does not hold true for the eyes of the horses, either living or dead, which were examined by us.
But in the case of some of the horses in whom Berlin had seen the phenomenon for which we sought in vain, he himself tells us, the deflection was very slight. In that case, it would appear, no great advantage would be gained along the lines indicated. But even assuming the degree of deflection to be very great, his theory goes to pieces on the very point it was supposed to explain. A concrete example will make this clear. If Mr. von Osten, standing two feet away from the horse, raised his head 1/5 millimeter (which figure by no means represents the extreme values that were obtained), then in the horse's retinal image every point of the man's head would move through a distance of 0.0025 millimeter—assuming the horse's eye to be free from astigmatism and assuming its focal distance to be 25.5 millimeters. If, however, other conditions remaining the same, we presuppose an extreme form of astigmatism, one in which the path of the retinal image is not a straight line, but is deflected into a semicircle, then each point would pass through a distance of nearly 0.004 millimeter. If the sensitive retinal elements have a diameter of 0.002 millimeter (as Berlin, somewhat inexactly, states), then from two to four elements would be stimulated in case there were no astigmatic deflection. But in case the deflection did take place, it would not necessarily involve more elements, as can be seen by making a simple graph; indeed we can imagine cases in which the circuitous path would involve even fewer elements than the straight one. And finally, when the movement which the horse is to perceive, does not occur in a straight line but in the form of a curve, (which will generally be the rule), then the astigmatism will tend in many cases to decrease the curvature of the image's path on the retina, and sometimes even obviate it entirely. In all these cases, on Berlin's own theory, the perception of the movements would be hindered rather than aided.[AA]
But to come now to the most pertinent objection. We saw that Berlin's whole train of thought rested upon the assertion that it made no difference whether we regarded by means of the speculum the seeming movement of a fixed retinal point, or whether the image of an external moving object is passing over the horse's retina. As a matter of fact, however, these two processes are very different from one another. In moving the mirror, with its small opening we are looking through ever changing portions of the horse's lens,—testing it out, as it were. The horse, on the other hand, sees with all parts of the lens simultaneously, in so far as the lens is not covered by the iris. The arcuate deflection, which is nothing but a registration of the difference in the indices of refraction of the different parts of the lens used consecutively, might thus be formed for the observer using the mirror, but never for the horse. For these reasons we cannot conclude that the kind of astigmatism described can really increase the horse's acuity in the perception of movements.
Since the light-refracting apparatus of the horse's eye does not offer a satisfactory explanation for the extraordinary keenness of visual perception possessed by the Osten horse, we must go a step further and ask whether it may not perhaps be found in the part immediately sensitive to light, the retina. That portion really would seem to be adapted to the perception of movements of minimal extent, and for this reason: it is more than three times as great in extent as the human retina, and the horse's retinal images are likewise larger owing to the position of the nodal point. The cells of the retina that are sensitive to light, the rods and cones, might therefore be correspondingly larger than those of the human eye, without thereby making the whole organ less efficient than the human eye. But the most recent measurements[51] have shown that the rods and cones of the horse's eye are more minute than ours. Assuming that, in the case of the horse, as is presumably the case in human vision, the transition of a stimulus from one retinal cell to the next already in itself induces a sensation of movement, then the horse ought indeed be extraordinarily keen in the perception of moving objects (provided that the horse's more minute cells are packed just as closely as in the human retina). And besides, there are two specially adapted areas within the retina of the horse. The "band"
("streifenförmige Area") which was discovered fifteen years ago by Chievitz,[52] is a strip of 1 to 1½ millimeters in width, traversing the entire retina horizontally, and is noteworthy on account of its structure and probably, too, on account of its greater efficiency. It may have something to do with the accomplishments of the Osten horse; but in how far it would be hard to say. The other noteworthy portion of the horse's retina is the "round area" discovered some four years ago, located at the rear outer end of the "band", and it is the best-equipped part of the horse's retina and corresponds to the area of clearest vision, the yellow spot, in the human eye. But this round area need not come in for consideration by us, for its location would indicate that it is used in binocular vision, that is, seeing with both eyes.[53] But in all our experiments the Osten horse observed only with one eye. That does not mean, however, that under other circumstances the round area may not be of very great importance.
In the present state of our knowledge, all attempts at explanation are, of course, of the nature of hypotheses. If further investigations should disclose this explanation to be untenable, then we would either have to suppose some unknown power in the eye of the horse,[AB] or else seek a cause in the animal's brain. Further experiments on other horses would be necessary in order to discover whether the species as a whole possesses this ability or whether only certain ones are thus endowed. The former is of course more probable. In this particular case conditions were unusually favorable for the development of this ability. We must bear in mind that in all probability Mr. von Osten's movements very gradually became as minute as they are now, and that therefore Hans at first learned to react to such as were relatively coarse. Furthermore, his practice extended throughout four years and during this time it was his sole occupation. Without specific predisposition, however, all this practice would have been utterly futile. We can also readily appreciate how indispensable in the struggle for existence a well-developed power of perceiving moving objects must be to horses (and most other animals) living in their natural condition and habitat, in order to be aware of the approach of enemies, or, in the case of carnivora, the presence of prey. In view of all these considerations we can readily see how it was possible that the horse, perhaps in spite of rather defective vision, could react with precision to movement-stimuli which escaped observation by human eyes.