Fresnel channel option click does not work on Mac. I can cut the entire transparency channel (aka revert to "None") with this -- but the ordinary selection offers the same options. If transparency is selected and there is a non-zero value in refraction channel, the fresnel term will also be called. Force drop, on the other hand -- good suggestion Patrick! -- does allow a single value to be placed in the channel even when refraction was present. Spent a while working with combinations of transparency value, fresnel term, refraction value or index. After 52 test renders did manage to get one in which the edge of my test sphere was visibly less transparent than the center. So it is do-able. If I had more reason I'd try the series again in a longer, more methodical exploration.