[Z] Since no opportunity was given us to examine Hans's eyes we do not know what their condition is in this respect. Though it would have been interesting to know, it would hardly make any difference in the views presented. If Hans should prove to be either far or near-sighted, then, if we are to make any supposition at all, it would be that the defect could not be very great, since near sightedness exceeding 2 or 3 diopters and far-sightedness exceeding one diopter is seldom found in the case of the horse. According to Mr. von Osten, Hans at one time manifested a tendency to shy easily. Be this as it may, for little could be concluded from it, since in many extremely shy horses, no kind of visual imperfection can be discovered.
[AA] For the benefit of specialists I would say the following in addition to the more general remarks just made. For the most part, the determinations of refraction made on the eye of the horse are still rather unreliable. In sciascopy there is a dispute among investigators concerning ambiguous shadows, and in the use of the refraction-ophthalmoscope no definite region of the eye's background has been adhered to by the various investigators. It appears that Riegel, whose diligent researches mentioned on [page 164] were published in 1904, knew nothing concerning the round area in the horse's eye, discovered by I. Zürn[42] in 1902. Also, if so great a degree of astigmatism is really the rule as is emphasized especially by Hirschberg[43] and Berlin,[44] then the simple refractive index usually given—sometimes within a half diopter—would be meaningless. Berlin[45] and Bayer[46] believe the vagueness of the retinal image resulting from the astigmatism, is offset by this: that the oval pupil functions as a stenopaic slit. In view of the width of the horse's pupil this appears to me to be rather hypothetical.
Concerning Berlin's theory of deflecting astigmatism I would say the following: Of the two ophthalmoscopic signs mentioned as being characteristic of this form of astigmatism,—the concentric circles and the arcuate deflection of the pathway of the fixated points,—when there is a movement of the eye of the observer (or of the eye observed), according to Berlin the former is not so constant as the latter. So far as I know, the concentric ring formation is mentioned only by Bayer[47] and Riegel,[48] and is said to occur principally in horses with myopic vision—and hence, relatively, in a minority of cases. Judging from the particulars, we are inclined to believe that a case of
"Butzenscheiben"-lens reported by Schwendimann[48a] is in reality a case of senile sclerosis. Berlin repeatedly warns us against mistaking the one for the other.[48b] The arcuate deflection, on the other hand, has not been mentioned elsewhere as a personal observation. In Berlin's calculation[49] of the increase in the extent of the retinal pathway an ambiguity has crept in. He says that "in the astigmatic eye there are stimulated 207 times as many nervous elements as would be stimulated in the ideally normal eye." It ought to read "207 more" instead of "207 times as many." And this number holds only for the one case computed by Berlin, and under the specific assumption that exactly π/2 times the normal number of elements were stimulated (571 instead of 364). Therefore the general statement which Bayer[50] makes in his text-book, that according to Berlin's evaluation "207 times more nervous elements" are stimulated in the astigmatic eye than in the non-astigmatic one, does not hold true.
Closing this note, a few remarks concerning the experiments made by Dr. Simon and myself. All of the nine horses were tested for the vertical image by means of the ophthalmoscope. In most cases Wolff's electric speculum was used. Atropine was not employed.—For the laboratory tests the adipose and the muscular tissues were removed from the eye-ball and the rear part of the bulb cut away. The front part, containing the cornea and the lens, was fastened over one opening of a metal cylinder which was closed at the other end by means of a disc of ground glass. The whole, approximately as long as a horse's eye, was filled with a normal salt solution whose refractive index (1.336) corresponds quite closely with that of the vitreous humor of the horse's eye. The pressure from within was regulated so that on the one hand it was not dimmed and yet on the other there were no wrinkles in the cornea. The source of light—the filament of a Nernst lamp—was moved about in a plane 120 cm. distant from the eye and perpendicular to the optic axis. It was moved through the point of intersection as well as at various distances from it. Movement in horizontal and vertical directions was in each case along lines 150 centimeters in length, which would correspond to an angle of vision of not less than 64°. The pathway of the imaged point was controlled by means of the cross-hairs of the telescope. If in the same way we observe through the sclerotic of an intact eye-bulb a point of light falling upon the retina and shining through the sclerotic and choroid (which is not difficult when we use an intense light), then to the observer its pathway will, of course, appear to be deflected convexly toward the periphery,—and the deflection will appear the greater, the farther the point of light is removed from the optic axis.
[AB] Königshöfer, who as we have already said, seconds the explanation given by the ophthalmologist Berlin (and who confounds "Butzenscheiben" astigmatism with the common, so-called regular form), believes[54] that not only astigmatism but also the shape of the blind-spot of the eye must be taken into consideration. This portion of the retina, where the fibres of the optic nerve enter the eye (and called "blind-spot" because there are no cells there that are sensitive to light) is very nearly circular in man, but differs in shape in the different species of animals. Königshöfer thought he had discovered that a relatively elongated blind spot was favorable to keenness of vision. If we place the mammalia in series on the basis of their relative keenness of vision, he says, we would find that this series is identical with the one in which they are grouped with reference to the form of the blind-spot from the circular up to the most elongated. (In such a series the marmot takes the place of honor.)
satisfactory, however. We cannot be sure what he means by "keenness of vision" ("scharfäugigkeit"). Is it visual acuity in the usual sense of the term (as is said in one of his passages), or keenness in the perception of the movements of objects, (this would appear to be his real meaning), or both at the same time. But whatever the significance he may put into the term, any such attempt at grouping the lower forms must prove unsatisfactory from the very start on account of the scant data which we possess on visual perception in animals. The experiences of the hunt upon which Königshöfer partly bases his view, are entirely inadequate for such a purpose. This much is certain, that the Osten horse, in spite of a blind-spot which,
though somewhat oval, is by no means very elongated, possesses an extraordinary acuity in the perception of movements. Even if the parallelism mentioned by Königshöfer were really shown to exist, it would not explain the matter until it were also shown in what way keenness of vision is dependent upon the shape of the blind-spot,—a portion of the eye which is not immediately operative in the visual sensation at all.
[AC] I can find examples of supposed suggestion in the case of animals given only by Rouhet.[61] He says that by means of suggestion he taught a half-year old half-blooded mare-colt which he had raised himself, to fetch and carry, and this in a very short time. In order to indicate to the colt what was wanted, Rouhet would concentrate with his whole mind upon the object intended (a watch), and at the same time he would bend forward slightly. In the third test, that is at the end of fifteen minutes, he had accomplished his purpose, and in the tenth lesson, no more mistakes occurred. The colt would fail to respond, however, as soon as he refrained from making any gestures, or was in a laissez faire frame of mind, or when he thought of other things. He therefore believes that there must have been some kind of immediate, though inexplicable, connection between the brain of the trainer and that of the horse. I think the explanation is evident: the connection was not as he thought, an immediate one, but arising through the mediation of the man's attitude ("attitude un peu baissée"), and of his movements ("gestes"), both resulting from his intense concentration ("tension de la pensée").