Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/7

 at a little distance to one side of the source of light and the reflected beam was made to illuminate the shadow falling on the white square, and itself to cast a shadow on the dark square. Thus each square was illuminated by the same light, but coming from different points, and the two would be caused to be of equal darkness by placing rotating sectors opening and closing at pleasure in front of one or other of the beams. The amount of light cut off gave a measure of the darkness. Thus,in one case, when both squares were white the sectors had a total aperture of 80° to make them equally luminous; when a grey platinum square was substituted for one of them, the aperture of the sectors was 45º. The grey surface, therefore, only reflected $45⁄80$ of the light that the white surface reflected. The light to these squares was admitted through an aperture in the front of a closed box, the illumination being judged through an aperture fixed at one corner. The whole apparatus was placed in a darkened and blackened room in which the sole light was that used for the measures.

Fig. 2.

The two following scales of blackness, or, perhaps more accurately, of whiteness, were obtained by this plan, one or other of which will apply to all the measurements given in the paper:-

Comparative intensities of light.

Curve I. Amount of white reflected.

Curve II. Amount of white reflected.