The most recent and by far the best edition of this work is that of H. M. Hine (Stuttgart/Leipzig, 1996), to which I refer for full bibliographical information. Many passages of the text are most helpfully discussed in the same scholar's Studies in the Text of Seneca's Naturales Quaestiones (Stuttgart/Leipzig, 1996).

ut nubes infici possint, … sol ad hoc apte ponendus est; non enim idem facit undecumque effulsit, et ad hoc opus est radiorum idoneus ictus.

Seneca is dealing with rainbows. Hine (Studies, 24–5) shares Axelson's suspicion of ictus, but is unhappy both with Axelson's situs (‘seems too static a word for the sun's rays’) and with my tractus, ‘direction’ (‘in this sense seems to be used only of concrete physical objects’); very tentatively he suggests angulus. Much more credible palaeographically and still yielding good sense and a good clausula would be i<mp>etus, a noun which is very common in this work of Seneca's.

Hostius fuit Quadra obscenitatis in scaenam usque perductae.