Lucretius is making a philosophical argument about how the images of visible objects are seen by our eyes.
nam cur illa cadant magis ab rebusque recedunt
quam quae tenvia sunt, hiscendist nulla potestas,
for why those should fall more from things and move away
than the ones that are thin, nobody can say.
I am reading "hiscendist nulla potestas" as:
"hiscendi est nulla potestas"
of answering there is no power
This parses "hiscendi" as a gerund, genitive singular.
And "hiscendist" is a contraction of "hiscendi est".
But this is guesswork.