On a Shadow in a Glass

By something form'd, I nothing am, Yet every thing that you can name; In no place have I ever been, Yet everywhere I may be seen; In all things false, yet always true, I'm still the same--but ever now. Lifeless, life's perfect form I wear, Can show a nose, eye, tongue, or ear, Yet neither smell, see, taste, nor hear. All shapes and features I can boast, No flesh, no bones, no blood-no ghost: All colors, without paint, put on, And change, like the chameleon. Swiftly I come, and enter there, Where not a chink lets in the air; Like thought, I'm in a moment gone, Nor can I ever be alone: All things on earth I imitate Faster than nature can create; Sometimes imperial robes I wear, Anon in beggar's rags appear; A giant now, and straight an elf, I'm every one, but ne'er myself; Ne'er sad I mourn, ne'er glad rejoice, I move my lips, but want a voice, I ne'er was born, nor ne'er can die, Then, pr'ythee, tell me what am I?