455.
Second condition.—If after breathing on a plate of glass, the breath is merely wiped away with the finger, and if we then again immediately breathe on the glass, we see very vivid colours gliding through each other; these, as the moisture evaporates, change their place, and at last vanish altogether. If this operation is repeated, the colours are more vivid and beautiful, and remain longer than they did the first time.
456.
Quickly as this appearance passes, and confused as it appears to be, I have yet remarked the following effects:—At first all the principal colours appear with their combinations; on breathing more strongly, the appearance may be perceived in some order. In this succession it may be remarked, that when the breath in evaporating becomes contracted from all sides towards the centre, the blue colour vanishes last.
457.
The phenomenon appears most readily between the minute lines, which the action of passing the fingers leaves on the clear surface; a somewhat rough state of the surface of the glass is otherwise requisite. On some glass the appearance may be produced by merely breathing; in other cases the wiping with the fingers is necessary: I have even met with polished mirror-glasses, one side of which immediately showed the colours vividly; the other not. To judge from some remaining pieces, the former was originally the front of the glass, the latter the side which was covered with quicksilver.