Sentence:  From above one could only occasionally catch a glimpse of life on the floor of this green sea : a neighbor 's gingham skirt flashing into sight for an <WSD>instant</WSD> on the path beneath her grape-arbor , or the movement of hands above a clothesline and the flutter of garments hung there , half-way down the block . 
Meaning of the word marked with <WSD> tags:  a very short time (as the time it takes the eye to blink or the heart to beat)