dim light

I changed a ceiling fixture in my bathroom and the light is very dim. its a 75 w bulb and its fine in other outlets but not bathroom ceiling. I'm reading 117 volts on my volt meter. its an old house (1954) with mostly cloth wiring. there is a constant hot there but its not hooked up to the switch.The light is fine when hooked to that wire. There is a newer black wire there that is hooked up to the switch and thats the one I have the issue with. If the voltage is there, why dismay light dim?