In the passage,
what is the meaning of さすれば? Is it the kana for 然すれば?
In this context, I would have expected 射せば to be written instead.
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さすれば is そうすれば (="if you do so", "then") said in an archaic fashion. And yes, さ here is 然, an archaic word that means そう in modern Japanese. This さ is almost dead now, but is found in a few set phrases like さも, さもありなん in modern Japanese.
(光を)射せば happens to make sense in this context, but it's not relevant.