Get the value of an instance variable given its name

In general, how can I get a reference to an object whose name I have in a string?

More specifically, I have a list of the parameter names (the member variables - built dynamically so I can't refer to them directly).

Each parameter is an object that also has an from_s method.

I want to do something like the following (which of course doesn't work...):

define_method(:from_s) do | arg |
    @ordered_parameter_names.each do | param |
        instance_eval "field_ref = @#{param}"
        field_ref.from_s(param)
    end
end