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 deg.. 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.]

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 desc