How magnificent the city is by the June moonlight! — after the streets are empty and silent.

The line contrasts the city’s daytime bustle with its nocturnal afterimage: once emptied of crowds and noise, the urban landscape can appear “magnificent,” transfigured by June moonlight. The speaker’s admiration is inseparable from absence—beauty emerges when commerce, traffic, and social demands recede. The seasonal marker (“June”) suggests warmth and late light, a time when one might linger outdoors and notice architectural contours, long shadows, and reflective surfaces. The dash and trailing clause (“— after…”) read like an afterthought that is actually the condition of the experience: the city becomes sublime not despite emptiness, but because silence allows perception and reverie.