in a line

References in classic literature ?
Hence, all my Flatland friends -- when I talk to them about the unrecognized Dimension which is somehow visible in a Line -- say, 'Ah, you mean BRIGHTNESS': and when I reply, 'No, I mean a real Dimension', they at once retort, 'Then measure it, or tell us in what direction it extends'; and this silences me, for I can do neither.