to rapidity at the outset, as it is quite clear that our eyes are accustomed to rapid changes of colour under ordinary circumstances. The rapidity of change in a tumbling sea under a brilliant sky is extreme, and yet we enjoy it thoroughly. So also is the rapidity of alteration of colour in moving objects and yet we can appreciate it; and in an express train the eye finds little difficulty in watching the rapid changes of colour presented to us by the landscape as it flies past us.
My early experiments led me to think that the difficulty was really due to alteration of intensity or luminosity of colour, rather than to that of tint or character of the colour, and it occurred to me that the effect of this alteration of intensity upon the eye might be greatly lessened if the colour-field upon the screen were surrounded by a background of pure white light. This acted successfully to a great extent, and in the newer forms of instrument two vertical bars or stripes of strong and pure white light are placed upon each side of the colour-field and opposed by black.
Experience has shown that in a very short
69
