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.