Having not read much Nabokov criticism yet, I am still mostly indebted to annotators online, and one astute contributor on genius, points out that that lemniscate "is a standard meteorological symbol for haze", connecting to the poet's desire for his daughter Hazel's soul to be eternal.
Haze generally is a phenomenon of obfuscation, a grey filter inhibiting vision and clarity, composed of dry particles such as smoke, dust, (stardust?) and sand (the latter a unit of time, a thousand years ago).