On first seeing (someone or something); immediately:
— It was love at first sight.

synonym: at first glance.

Roget 441: visibly etc. 446; in sight of, with one's eyes open at sight, at first sight, at a glance, at the first blush; prima facie [Lat.].   

Roget 448: apparently; to all seeming, to all appearance; ostensibly, seemingly, as it seems, on the face of it, prima facie [Lat.]; at the first blush, at first sight; in the eyes of; to the eye.   

After an initial impression, which later proves incorrect.

synonym: at first glance.