This is the more probable as the investigation was carried out under less favourable conditions than was our later work, and with less satisfactory apparatus, the new Oxford Psycho- Physical Laboratory not being in existence at that time.

Owing to these difficulties, the work was discontinued at the time and not taken up again until the spring of 1909, when it was continued in the new laboratory with the apparatus and arrangements already described.

In the first series undertaken under these new conditions the 13mm.

Aperture through which the light entered the dark room was left uncovered so that the ray from the lantern was projected directly into the eye of the observer during the passage of the open sectors of the disc.

In the following table, W represents the width in degrees of the wider open sector, N that of the narrower sector.