used for repeating the experiments which show the strengthening of the action on the retina by light when "N" rays are acting simultaneously on the eye.
These last experiments may be made in a very simple manner. The shutters of a room should be closed so as to leave just enough light for a white surface standing out on a dark background—for instance, the dial of a clock—to appear, before an observer 4 or 5 metres distant, like a grey patch with ill-defined contours. If a cane stick placed before the eyes is bent, the grey surface is seen to whiten; if the cane is allowed to straighten, the surface grows dark again. Instead of the cane, a slip of plate-glass can be used. This is bent either with the press employed in lectures for showing the doubly refractive property of glass acquired by flexure, or simply with the hands. With a suitable amount of light, which may be obtained after a few trials, these phenomena are easily visible. They are not instantaneous, as I have already explained. It is of the utmost importance that this retardation be taken into account when one wishes to study these phenomena;
