Both are therefore strictly questions of quality.  And the two are sufficient to identify appearance 5, for if no red or orange is visible, images 1, 2, and 3 are excluded; and if no green lies to the right or left of the yellow band, image 4 is excluded.  Thus if one is to make the somewhat superficial distinction between qualitative and quantitative judgments, the judgments here required are qualitative.  Moreover, the subjects make these judgments unhesitatingly.

Finally, the method of making judgments on after-images is not new in psychology.  Lamansky’s well-known determination of the rate of eye-movements[22] depends on the possibility of counting accurately the number of dots in a row of after-images.  A very much bolder assumption is made by Guillery[23] in another measurement of the rate of eye-movements.  A trapezoidal image was generated on the moving retina, and the after-image of this was projected on to a plane bearing a scale of lines inclining at various angles.  On this the degree of inclination of one side of the after-image was read off, and thence the speed of the eye-movement was calculated.  In spite of the boldness of this method, a careful reading of Guillery’s first article cited above will leave no doubt as to its reliability, and the accuracy of discrimination possible on these after-images.

   [22] Lamansky, S., (Pflueger’s) Archiv f. d. gesammte
   Physiologie, 1869, II., S. 418.

   [23] Guillery, (Pflueger’s) Archiv f. d. ges.  Physiologie, 1898,
   LXXI., S. 607; and 1898, LXXIII., S. 87.

As to judgments on the color and color-phases of after-images, there is ample precedent in the researches of von Helmholtz, Hering, Hess, von Kries, Hamaker, and Munk.  It is therefore justifiable to assume the possibility of making accurately the four simple judgments of shape and color described above, which are essential to the two proofs of anaesthesia.

V. SUMMARY AND COROLLARIES OF THE EXPERIMENTS, AND A PARTIAL, PHYSIOLOGICAL INTERPRETATION OF THE CENTRAL ANAESTHESIA.

We have now to sum up the facts given by the experiments.  The fact of central anaesthesia during voluntary movement is supported by two experimental proofs, aside from a number of random observations which seem to require this anaesthesia for their explanation.  The first proof is that if an image of the shape of a dumb-bell is given to the retina during an eye-movement, and in such a way that the handle of the image, while positively above the threshold of perception, is yet of brief enough duration to fade completely before the end of the movement, it then happens that both ends of the dumb-bell are seen but the handle not at all.  The fact of its having been properly given to the retina is made certain by the presence of the now disconnected ends.