The contrast associated with dislocations in deformed $\alpha$-iron has been examined under a large number of two-beam diffracting conditions using transmission electron microscopy. Dislocations are shown to go effectively out of contrast for several different diffracting vectors (g), and the angular relations between these vectors are often such that the observed absence of contrast cannot always be associated with the condition g.b=0, where b is the Burgers vector of the dislocation. Images have been computed for the experimental conditions used and it is shown that under certain two-beam conditions dislocations are predicted to be invisible when g.b $\neq$ 0. It is shown that the explanation for this behaviour lies in the large values of s$_g$, the deviation parameter, which are usually associated with large diffracting vectors. It is concluded that erroneous deductions of the Burgers vectors of dislocations have been made previously because this absence of contrast has been wrongly assumed to correspond to the condition g.b = 0.