acquainted with the true nature of this phenomenon.
412.
But since in these experiments everything depends on our being persuaded of the parallactic action of the light, we can make this more evident by means of two sources of light, the two shadows from which intersect each other, and may be altogether separated. By day this may be contrived with two small openings in a window-shutter; by night, with two candles. There are even accidental effects in interiors, on opening and closing shutters, by means of which we can better observe these appearances than with the most careful apparatus. But still, all and each of these may be reduced to experiment by preparing a box which the observer can look into from above, and gradually diminishing the openings after having caused a double light to shine in. In this case, as might be expected, the coloured shadow, considered under the physiological colours, appears very easily.
413.
It is necessary to remember, generally, what has been before stated with regard to the nature of double shadows, half-lights, and the like. Experiments also should especially be made with different shades of grey placed next each other, where every stripe will appear light by a darker, and dark by a lighter stripe next it. If at night, with three or more lights, we produce shadows which cross each other successively, we can observe this phenomenon very distinctly, and we shall be convinced that the physiological case before more fully treated, here comes into the account (38).
414.
To what extent