Sentence:  It is generally believed that the planets do not <WSD>scintillate</WSD> at all, or scarcely at all. Nevertheless I have often observed a sensible scintillation of Venus and Mars, and in a few rare cases I have also observed a slight scintillation of Jupiter and Saturn. ... I would therefore call the attention of observers who may find themselves under atmospherical conditions of a nature to render the general scintillation very strong, to this point, as they might perhaps be able to ascertain whether Jupiter and Saturn ever sensibly <WSD>scintillate</WSD>. 
Meaning of the word marked with <WSD> tags:  Of a star or other celestial body: to vary rapidly in brightness; to twinkle.