Note AA — Chapter V · EXPLANATION OF THE OBSERVATIONS

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 [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.