One of the first things I wanted to know about vertical disparity was how finely we are able to resolve it. This had been studied in a previous paper by Kaneko & Howard (1997, Vision Research 37 (20): 2871-2878), but there were a number of issues which made me feel the question hadn't been fully resolved. We used essentially the same stimulus as Kaneko & Howard: that is, an alternating version of Ogle's induced effect, in which vertical magnification alternated in strips across the image. We assumed that the task would be easiest when the magnification was constant across the whole image, and become progressively harder as the strip-width was reduced. To our surprise, this wasn't the case for most subjects. About a third of our observers did behave in this way, but most of us found that the abrupt switches in the sign of vertical magnification were quite salient and actually helped us do the task -- resulting in a band-pass, rather than a low-pass, performance profile. I have worried a lot about whether this result could be due to some artefact, but we've tested for everything we can think of and it keeps showing up time after time. So, there must be mechanisms in the brain which respond to discontinuities in vertical disparity. The existing literature emphasises the continuity of vertical disparity in natural viewing. But actually, discontinuities can occur under some circumstances, so it is possible that we have developed detectors for these.